Fixes #3604.

This PR relaxes our rules to allow default initializers to be present on required parameters as well as optional parameters. To be concrete, we allow default initializers on all parameters. To be considered an optional parameter, the parameter must have a question mark, or it must have an initializer and not be followed by a required parameter anywhere in the rest of the signature.

For example, in the following signatures, a is optional:

(a = 0, b = 0);
(a = 0, b?);
(a = 0, ...b);

But in the following signature, a is not optional:

(a = 0, b);

One thing to note is that an optional parameter corresponds to an argument slot that is allowed to be missing. The default initializers for required parameters kick in when undefined is passed, but the argument is not allowed to be omitted entirely.

This should not be a breaking change.