Editor: Guard entity preload filters and share entity actions/fields logic - #14716

`_wp_preload_entity_actions()` and `_wp_preload_entity_fields()` assumed
`$paths` was an array, so the editor crashed when an earlier
`block_editor_rest_api_preload_paths` callback returned `null`, `false`,
a string or an object. Empty values are now turned into an array so the
entity paths are still added. Other non-array values, and contexts that
are not a `WP_Block_Editor_Context`, are returned unchanged. `$context`
now defaults to `null`, so calling either function with one argument no
longer throws an `ArgumentCountError`.

Numeric script module ids are now handled as strings. PHP turned them into
integer array keys, so `wp_get_registered_entity_field_modules()` and
`wp_get_registered_entity_action_modules()` could miss them in a strict
comparison and `array_merge()` could renumber them. Both functions now
return a list of entries holding the module id and the ids of the fields
or actions it applies to. This is the shape the REST controllers already
returned, and they now use these results directly. The entity fields
endpoint now returns module ids as strings, matching its schema.

A field registered with the id `'0'` is no longer rejected as empty.

The code duplicated between the two APIs now lives in a new
`entity-utils.php` file:
- `_wp_add_entity_preload_paths()` builds the preload paths for both
  preload callbacks.
- `_wp_add_entity_modules_to_editor_script()` adds the script modules to
  the editor script for both `admin_footer` callbacks. It takes a callable
  rather than the modules, so the registries are still only read on pages
  that load the editor script.

Developed in #14716.

Props oandregal, jorgefilipecosta.
See #66229, #66343.


git-svn-id: https://develop.svn.wordpress.org/trunk@64686 602fd350-edb4-49c9-b593-d223f7449a82

`_wp_preload_entity_actions()` and `_wp_preload_entity_fields()` assumed
`$paths` was an array, so the editor crashed when an earlier
`block_editor_rest_api_preload_paths` callback returned `null`, `false`,
a string or an object. Empty values are now turned into an array so the
entity paths are still added. Other non-array values, and contexts that
are not a `WP_Block_Editor_Context`, are returned unchanged. `$context`
now defaults to `null`, so calling either function with one argument no
longer throws an `ArgumentCountError`.

Numeric script module ids are now handled as strings. PHP turned them into
integer array keys, so `wp_get_registered_entity_field_modules()` and
`wp_get_registered_entity_action_modules()` could miss them in a strict
comparison and `array_merge()` could renumber them. Both functions now
return a list of entries holding the module id and the ids of the fields
or actions it applies to. This is the shape the REST controllers already
returned, and they now use these results directly. The entity fields
endpoint now returns module ids as strings, matching its schema.

A field registered with the id `'0'` is no longer rejected as empty.

The code duplicated between the two APIs now lives in a new
`entity-utils.php` file:
- `_wp_add_entity_preload_paths()` builds the preload paths for both
  preload callbacks.
- `_wp_add_entity_modules_to_editor_script()` adds the script modules to
  the editor script for both `admin_footer` callbacks. It takes a callable
  rather than the modules, so the registries are still only read on pages
  that load the editor script.

Developed in WordPress/wordpress-develop#14716.

Props oandregal, jorgefilipecosta.
See #66229, #66343.

Built from https://develop.svn.wordpress.org/trunk@64686


git-svn-id: http://core.svn.wordpress.org/trunk@63836 1a063a9b-81f0-0310-95a4-ce76da25c4cd