Themes: Register the block-templates theme feature.

This ensures the feature is included in the `theme_supports` REST API data for the active theme.

Follow-up to [48171].

Props grantmkin, TimothyBlynJacobs, addiestavlo.
Fixes #55681.

git-svn-id: https://develop.svn.wordpress.org/trunk@53659 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -3804,6 +3804,7 @@ function _wp_keep_alive_customize_changeset_dependent_auto_drafts( $new_status,
  * See {@see 'setup_theme'}.
  *
  * @since 5.5.0
+ * @since 6.1.0 The `block-templates` feature was added.
  */
 function create_initial_theme_features() {
 	register_theme_feature(
@@ -3820,6 +3821,13 @@ function create_initial_theme_features() {
 			'show_in_rest' => true,
 		)
 	);
+	register_theme_feature(
+		'block-templates',
+		array(
+			'description'  => __( 'Whether a theme uses block-based templates.' ),
+			'show_in_rest' => true,
+		)
+	);
 	register_theme_feature(
 		'custom-background',
 		array(