feat(forms): add updateOn blur option to FormControls - #18408

This commit introduces a new option to template-driven forms that
improves performance by delaying form control updates until the
"blur" or "submit" event.  To use it, set the `updateOn` property
in `ngModelOptions`.

```html
<input ngModel [ngModelOptions]="{updateOn: blur}">
```

Like in AngularJS, setting `updateOn` to `blur` or `submit` will
delay the update of the value as well as the validation status.
Updating value and validity together keeps the system easy to reason
about, as the two will always be in sync.  It's also worth noting
that the value/validation pipeline does still run when the form is
initialized (in order to support initial values).

Upcoming PRs will address:

* Support for setting group-level `updateOn` in template-driven forms
* Option for skipping initial validation run or more global error
display configuration
* Better support of reactive validation strategies

See more context in angular#18408, angular#18514, and the [design doc](https://docs.google.com/document/d/1dlJjRXYeuHRygryK0XoFrZNqW86jH4wobftCFyYa1PA/edit#heading=h.r6gn0i8f19wz).

By default, the value and validation status of a `FormControl` updates
whenever its value changes. If an application has heavy validation
requirements, updating on every text change can sometimes be too expensive.

This commit introduces a new option that improves performance by delaying
form control updates until the "blur" event.  To use it, set the `updateOn`
option to `blur` when instantiating the `FormControl`.

```ts
// example without validators
const c = new FormControl(, { updateOn: blur });

// example with validators
const c= new FormControl(, {
   validators: Validators.required,
   updateOn: blur
});
```

Like in AngularJS, setting `updateOn` to `blur` will delay the update of
the value as well as the validation status. Updating value and validity
together keeps the system easy to reason about, as the two will always be
in sync. It's  also worth noting that the value/validation pipeline does
still run when the form is initialized (in order to support initial values).

Closes angular#7113

This commit introduces a new option to template-driven forms that
improves performance by delaying form control updates until the
"blur" or "submit" event.  To use it, set the `updateOn` property
in `ngModelOptions`.

```html
<input ngModel [ngModelOptions]="{updateOn: blur}">
```

Like in AngularJS, setting `updateOn` to `blur` or `submit` will
delay the update of the value as well as the validation status.
Updating value and validity together keeps the system easy to reason
about, as the two will always be in sync.  It's also worth noting
that the value/validation pipeline does still run when the form is
initialized (in order to support initial values).

Upcoming PRs will address:

* Support for setting group-level `updateOn` in template-driven forms
* Option for skipping initial validation run or more global error
display configuration
* Better support of reactive validation strategies

See more context in #18408, #18514, and the [design doc](https://docs.google.com/document/d/1dlJjRXYeuHRygryK0XoFrZNqW86jH4wobftCFyYa1PA/edit#heading=h.r6gn0i8f19wz).

By default, the value and validation status of a `FormControl` updates
whenever its value changes. If an application has heavy validation
requirements, updating on every text change can sometimes be too expensive.

This commit introduces a new option that improves performance by delaying
form control updates until the "blur" event.  To use it, set the `updateOn`
option to `blur` when instantiating the `FormControl`.

```ts
// example without validators
const c = new FormControl(, { updateOn: blur });

// example with validators
const c= new FormControl(, {
   validators: Validators.required,
   updateOn: blur
});
```

Like in AngularJS, setting `updateOn` to `blur` will delay the update of
the value as well as the validation status. Updating value and validity
together keeps the system easy to reason about, as the two will always be
in sync. It's  also worth noting that the value/validation pipeline does
still run when the form is initialized (in order to support initial values).

Closes angular#7113

This commit introduces a new option to template-driven forms that
improves performance by delaying form control updates until the
"blur" or "submit" event.  To use it, set the `updateOn` property
in `ngModelOptions`.

```html
<input ngModel [ngModelOptions]="{updateOn: blur}">
```

Like in AngularJS, setting `updateOn` to `blur` or `submit` will
delay the update of the value as well as the validation status.
Updating value and validity together keeps the system easy to reason
about, as the two will always be in sync.  It's also worth noting
that the value/validation pipeline does still run when the form is
initialized (in order to support initial values).

Upcoming PRs will address:

* Support for setting group-level `updateOn` in template-driven forms
* Option for skipping initial validation run or more global error
display configuration
* Better support of reactive validation strategies

See more context in angular#18408, angular#18514, and the [design doc](https://docs.google.com/document/d/1dlJjRXYeuHRygryK0XoFrZNqW86jH4wobftCFyYa1PA/edit#heading=h.r6gn0i8f19wz).

By default, the value and validation status of a `FormControl` updates
whenever its value changes. If an application has heavy validation
requirements, updating on every text change can sometimes be too expensive.

This commit introduces a new option that improves performance by delaying
form control updates until the "blur" event.  To use it, set the `updateOn`
option to `blur` when instantiating the `FormControl`.

```ts
// example without validators
const c = new FormControl(, { updateOn: blur });

// example with validators
const c= new FormControl(, {
   validators: Validators.required,
   updateOn: blur
});
```

Like in AngularJS, setting `updateOn` to `blur` will delay the update of
the value as well as the validation status. Updating value and validity
together keeps the system easy to reason about, as the two will always be
in sync. It's  also worth noting that the value/validation pipeline does
still run when the form is initialized (in order to support initial values).

Closes angular#7113

This commit introduces a new option to template-driven forms that
improves performance by delaying form control updates until the
"blur" or "submit" event.  To use it, set the `updateOn` property
in `ngModelOptions`.

```html
<input ngModel [ngModelOptions]="{updateOn: blur}">
```

Like in AngularJS, setting `updateOn` to `blur` or `submit` will
delay the update of the value as well as the validation status.
Updating value and validity together keeps the system easy to reason
about, as the two will always be in sync.  It's also worth noting
that the value/validation pipeline does still run when the form is
initialized (in order to support initial values).

Upcoming PRs will address:

* Support for setting group-level `updateOn` in template-driven forms
* Option for skipping initial validation run or more global error
display configuration
* Better support of reactive validation strategies

See more context in angular#18408, angular#18514, and the [design doc](https://docs.google.com/document/d/1dlJjRXYeuHRygryK0XoFrZNqW86jH4wobftCFyYa1PA/edit#heading=h.r6gn0i8f19wz).