wp_theme_has_theme_json: add public method to know whether the theme supports theme.json

[oandregal]

Trac ticket https://core.trac.wordpress.org/ticket/56975

Related Gutenberg PRs:

• Add wp_theme_has_theme_json as a public API to know whether a theme has a theme.json gutenberg#45168
• Deprecate WP_Theme_JSON_Resolver:theme_has_support() gutenberg#45380
• Make theme.json object caches non persistent gutenberg#46150
• Don't check if constants set by wp_initial_constants() are defined gutenberg#45979
• Cleaner logic in wp_theme_has_theme_json gutenberg#45950

Lnking to Gutenberg's 6.2 backport tracking ticket.

What

Adds a new public method wp_theme_has_theme_json that checks whether a theme has a theme.json file.

Why

Part of WordPress/gutenberg#45171

This offers consumers a public API so they don't have to rely on private code. Additionally, it uses the object cache under the theme_json group to maintain this data, consolidating with the changes done for settings (core PR) and settings (core PR soon, see Gutenberg PR instead).

How

Creates the new public method, substitutes usage, and deprecates the old one.

Test

• Verify that automated tests pass.
• Using TwentyTwentyThree (a theme with a theme.json) and then a TwentyTwentyOne (a theme without theme.json) do the following:
  • Load the front-end. Verify that styles load as expected.
  • Load the post editor. Verify that the presets (e.g.: color) are set correctly in the block inspector.