REST API: Support subdirectory themes in the Themes controller.

This allows for themes that are included inside of a subdirectory, for example `subdir/my-theme`, to be accessed via the single item route of the `/wp/v2/themes` controller.

Fixes #54349.


git-svn-id: https://develop.svn.wordpress.org/trunk@52017 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php

@@ -16,6 +16,8 @@
  */
 class WP_REST_Themes_Controller extends WP_REST_Controller {
 
+	const PATTERN = '[^.\/]+(?:\/[^.\/]+)?';
+
 	/**
 	 * Constructor.
 	 *
@@ -50,7 +52,7 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 
 		register_rest_route(
 			$this->namespace,
-			'/' . $this->rest_base . '/(?P<stylesheet>[\w-]+)',
+			sprintf( '/%s/(?P<stylesheet>%s)', $this->rest_base, self::PATTERN ),
 			array(
 				'args'   => array(
 					'stylesheet' => array(