Add __experimentalFeatures data for the block editor

[oandregal]

Part of https://core.trac.wordpress.org/ticket/53175

This PR ports the first piece of landing the theme.json processing in WordPress core. It deals with merging the new settings (__experimentalFeatures) for all editors to access. The second piece would be to land the stylesheet engine.

What this PR does

• Adds a new setting __experimentalFeatures to the block editor settings. This data comes from theme.json (core and theme). See for the whole rationale about the theme.json file.
• Adds a theme-i18n.json file to WordPress core, to be used by core, the plugin, and wp-cli for translations.
• Adds the necessary code to make this work (namely the WP_Theme_JSON and WP_Theme_JSON_Resolver classes. These are used to pull settings from theme.json (core and theme). If you compare these with the ones that come with Gutenberg you'll note that this PR only includes the necessary bits for settings to work. Subsequent PRs will deal with adding styles, etc.

How this can be extended

There are a few ways:

• For major changes (change how core and theme data is merged, changes to the shape of theme.json file, etc): because we're adding new data under settings.__experimentalFeatures this can be filtered by the existing block editor filters.
• To add more settings and its default values: two new filters are introduced.
  • theme_json_allowed_schema: to modify the schema used in sanitize the theme.json data.
  • theme_json_core_data: to set defaults for core data, including new settings added via the previous filter.

Related work

Dependant PRs:

• Add _wp_array_set function and extract it from this PR Add _wp_array_set function #1263 (to land first)
• Adapt Gutenberg for this changes Adapt Gutenberg code to work after global styles lands in core gutenberg#32059

Follow-ups:

• Prepare a similar PR to add the styles engine.
• Port the changes to the merge algorithm introduced at Make core colors classes and custom properties always available gutenberg#31669 when the Gutenberg 10.7 packages are ported to core.

How to test

A theme without theme.json:

• Activate the TwentyTwentyOne theme (doesn't have a theme.json).
• Go to the editor and make sure that there's a new key __experimentalFeatures available under settings in the core/block-editor store and that it contains the ALLOWED_SETTINGS (see)
• A good litmus test is that the color palette is shown properly in the UI controls. Verify it's the same defined by the theme.

A theme with theme.json support (but doesn't provide specific color palette):

• Install and activate the emptytheme.
• Go to the editor and make sure that there's a new key __experimentalFeatures available under settings in the core/block-editor store and that it contains the ALLOWED_SETTINGS (see)
• Check that the color palette shown is the one by core.

A theme with theme.json support (but does provide a color palette):

• Install and activate the TT1-blocks theme.
• Go to the editor and make sure that there's a new key __experimentalFeatures available under settings in the core/block-editor store and that it contains the ALLOWED_SETTINGS (see)
• Check that the color palette shown is the one by the theme.

With the Gutenberg plugin active:

• Use this branch Adapt Gutenberg code to work after global styles lands in core gutenberg#32059
• Go to the editor and make sure that there's a new key __experimentalFeatures available under settings in the core/block-editor store and that it contains the allowed settings passed from the plugin (see gutenberg_global_styles_theme_json_allowed_settings method).

[oandregal]

Extracted this to #1263 Added here for simplicity of review/test. That PR should land before this one and should be reviewed over there.

[oandregal]

This file comes from https://github.com/WordPress/gutenberg/blob/trunk/lib/class-wp-theme-json-resolver.php but I've removed the bits that weren't necessary just yet (user data, etc).

[oandregal]

This file comes from https://github.com/WordPress/gutenberg/blob/trunk/lib/class-wp-theme-json.php and I've removed the bits that weren't necessary just yet (processing styles, settings that are not stable yet so aren't mean to land in core, etc).

[oandregal]

Current status: the code has been adapted from Gutenberg's to only contain the parts that deal with the settings. All tests are passing. It's functional using themes with and without theme.json support. Gutenberg can be activated using this companion PR WordPress/gutenberg#32059

[oandregal]

Note that this function hasn't been updated to include the changes to the merge algorithm introduced at WordPress/gutenberg#31669 It needs the 10.7 Gutenberg packages to work as expected. It should be updated later.

[youknowriad]

Is there a way to mark all these introduced classes as "internal" to allow them to refactor/update the code in future versions?

[oandregal]

I don't know. I've seen some function names prepended as _function_name when they're only expected to be used internally. Can we do the same for the classes or using something like _EXPERIMENTAL_?

[youknowriad]

Maybe we can just mark it @access private or something like that. @desrosj would know maybe?

[oandregal]

Done that at 665801c

[youknowriad]

I think this is a very good first step. I'm just concerned about the number of new APIs this exposes in Core. I'd rather have just "theme.json" file (for themes) and block_editor_settings hooks for plugins as APIs, all the rest I'd be more confident if it stays as internal implementation for now.

[oandregal]

Landed at ef3da33

[oandregal]

As per the follow-ups on this PR:

• Prepare a similar PR to add the styles engine. Add global styles #1287
• Port the changes to the merge algorithm introduced at Make core colors classes and custom properties always available gutenberg#31669 when the Gutenberg 10.7 packages are ported to WordPress core. Update color merging algorithm #1289 (necessary since the packages were published this morning).