Themes: Add internal-only theme.json's webfonts handler (stopgap).

Adds `_wp_theme_json_webfonts_handler()` for handling `fontFace` declarations in a theme's `theme.json` file to generate the `@font-face` styles for both the editor and front-end.

Design notes:
* It is not a public API, but rather an internal, Core-only handler.
* It is a stopgap implementation that will be replaced when the public Webfonts API is introduced in Core.
* The code design is intentional, albeit funky, with the purpose of avoiding backwards-compatibility issues when the public Webfonts API is introduced in Core.
   * It hides the inter-workings.
   * Does not exposing API ins and outs for external consumption.
   * Only works for `theme.json`.
   * Does not provide registration or enqueuing access for plugins.

For more context on the decision to include this stopgap and the Webfonts API, see:
* Core's PR 40493 WordPress/gutenberg#40493
* Gutenberg's tracking issue 40472 WordPress/gutenberg#40472

Props aristath, hellofromTonya, peterwilsoncc, costdev, jffng, zieladam, gziolo, bph, jonoaldersonwp, desrosj.

See #55567, #46370.

git-svn-id: https://develop.svn.wordpress.org/trunk@53282 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

src/wp-includes/default-filters.php

@@ -351,6 +351,7 @@
 add_action( 'after_switch_theme', '_wp_menus_changed' );
 add_action( 'after_switch_theme', '_wp_sidebars_changed' );
 add_action( 'wp_print_styles', 'print_emoji_styles' );
+add_action( 'plugins_loaded', '_wp_theme_json_webfonts_handler' );
 
 if ( isset( $_GET['replytocom'] ) ) {
 	add_filter( 'wp_robots', 'wp_robots_no_robots' );