REST API: Indicate when a theme supports the Site editor in the Themes REST API response.

This changeset adds a `is_block_theme` property to each theme in the `wp/v2/themes` API response, which uses `WP_Theme::is_block_theme` to determinate whether 
the theme is block theme or not.

Props grantmkin, ironprogrammer, zunaid321, azaozz, spacedmonkey, audrasjb, costdev.
Fixes #58123.




git-svn-id: https://develop.svn.wordpress.org/trunk@55951 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -326,6 +326,10 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 			}
 		}
 
+		if ( rest_is_field_included( 'is_block_theme', $fields ) ) {
+			$data['is_block_theme'] = $theme->is_block_theme();
+		}
+
 		$data = $this->add_additional_fields_to_object( $data, $request );
 
 		// Wrap the data in a response object.
@@ -494,6 +498,11 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 						),
 					),
 				),
+				'is_block_theme' => array(
+					'description' => __( 'Whether the theme is a block-based theme.' ),
+					'type'        => 'boolean',
+					'readonly'    => true,
+				),
 				'name'           => array(
 					'description' => __( 'The name of the theme.' ),
 					'type'        => 'object',