Themes: Introduce wp_theme_has_theme_json() for public consumption.

Adds `wp_theme_has_theme_json()` for public consumption, to replace the private internal Core-only `WP_Theme_JSON_Resolver::theme_has_support()` method. This new global function checks if a theme or its parent has a `theme.json` file.

For performance, results are cached as an integer `1` or `0` in the `'theme_json'` group with `'wp_theme_has_theme_json'` key. This is a non-persistent cache. Why? To make the derived data from `theme.json` is always fresh from the potential modifications done via hooks that can use dynamic data (modify the stylesheet depending on some option, settings depending on user permissions, etc.).

Also adds a new public function `wp_clean_theme_json_cache()` to clear the cache on `'switch_theme'` and `start_previewing_theme'`.

References:
* [https://github.com/WordPress/gutenberg/pull/45168 Gutenberg PR 45168] Add `wp_theme_has_theme_json` as a public API to know whether a theme has a `theme.json`.
* [https://github.com/WordPress/gutenberg/pull/45380 Gutenberg PR 45380] Deprecate `WP_Theme_JSON_Resolver:theme_has_support()`.
* [https://github.com/WordPress/gutenberg/pull/46150 Gutenberg PR 46150] Make `theme.json` object caches non-persistent.
* [https://github.com/WordPress/gutenberg/pull/45979 Gutenberg PR 45979] Don't check if constants set by `wp_initial_constants()` are defined.
* [https://github.com/WordPress/gutenberg/pull/45950 Gutenberg PR 45950] Cleaner logic in `wp_theme_has_theme_json`.

Follow-up to [54493], [53282], [52744], [52049], [50959].

Props oandregal, afragen, alexstine, aristath, azaozz, costdev, flixos90, hellofromTonya, mamaduka, mcsf, ocean90, spacedmonkey.
Fixes #56975.

git-svn-id: https://develop.svn.wordpress.org/trunk@55086 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/load.php

@@ -753,7 +753,7 @@ function wp_start_object_cache() {
 			)
 		);
 
-		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins' ) );
+		wp_cache_add_non_persistent_groups( array( 'counts', 'plugins', 'theme_json' ) );
 	}
 
 	$first_init = false;