Add method for plugins to register theme style variations

[justintadlock]

What problem does this address?

Currently, only themes can "register" theme style variations by adding JSON files to their /styles folder. WP_Theme_JSON_Resolver::get_style_variations() does this by scanning the folder and looking for the files in the parent and child themes: https://developer.wordpress.org/reference/classes/wp_theme_json_resolver/get_style_variations/

However, there are no filter hooks or an API for adding custom variations outside of using the /styles folder.

Related support forum issue: https://wordpress.org/support/topic/custom-wordpress-styles-variations-for-block-themes-using-plugin/

What is your proposed solution?

There are several methods that might be worth exploring:

• Create an API for registering theme.json-like style data via PHP (similar to what's possible with block style variations and register_block_style()).
• Add a filter hook over the result of WP_Theme_JSON_Resolver::get_style_variations() to let plugins filter the variations.
• Add a filter hook for the directory paths that should be scanned for JSON files.

Each of these methods could also potentially solve #51277

[bph]

There was also a support request as well: How to delete a (style combination) directly with the plugin