Themes: Add static cache variable to wp_theme_has_theme_json().

For performance, a static variable is added to `wp_theme_has_theme_json()` to cache the boolean result of determining if a theme (or its parent) has a `theme.json` file. 

This cache avoids the overhead of calling `get_stylesheet_directory()` and `get_template_directory()` each time `wp_theme_has_theme_json()` is invoked.

The cache is lean, non-persistent, and encapsulated within the function (i.e. function scope and not available externally).

The cache is ignored when:
* `WP_DEBUG` is on to avoid interrupting theme developer's workflow and for extender automated test suites.
* `WP_RUN_CORE_TESTS` is on to ensure each Core test exercises the checking code.

Follow-up to [55092], [55086].

Props oandregal, azaozz, costdev, dmsnell, flixos90, hellofromTonya, Otto42, spacedmonkey.
Fixes #56975.
Built from https://develop.svn.wordpress.org/trunk@55138


git-svn-id: http://core.svn.wordpress.org/trunk@54671 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/global-styles-and-settings.php

@@ -264,6 +264,26 @@ function wp_add_global_styles_for_blocks() {
  * @return bool Returns true if theme or its parent has a theme.json file, false otherwise.
  */
 function wp_theme_has_theme_json() {
+	static $theme_has_support = null;
+
+	if (
+		null !== $theme_has_support &&
+		/*
+		 * Ignore static cache when `WP_DEBUG` is enabled. Why? To avoid interfering with
+		 * the theme developer's workflow.
+		 *
+		 * @todo Replace `WP_DEBUG` once an "in development mode" check is available in Core.
+		 */
+		! WP_DEBUG &&
+		/*
+		 * Ignore cache when automated test suites are running. Why? To ensure
+		 * the static cache is reset between each test.
+		 */
+		! ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS )
+	) {
+		return $theme_has_support;
+	}
+
 	// Does the theme have its own theme.json?
 	$theme_has_support = is_readable( get_stylesheet_directory() . '/theme.json' );
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55137';
+$wp_version = '6.2-alpha-55138';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.