Block Editor: Introduce block templates for classic themes.

With this patch, users will be able to create custom block based templates
and assign them to specific pages/posts.

Themes can also opt-out of this feature

Props bernhard-reiter, carlomanf.
Fixes #53176.


git-svn-id: https://develop.svn.wordpress.org/trunk@51003 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/taxonomy.php

@@ -172,6 +172,25 @@ function create_initial_taxonomies() {
 			'show_in_nav_menus' => current_theme_supports( 'post-formats' ),
 		)
 	);
+
+	register_taxonomy(
+		'wp_theme',
+		array( 'wp_template' ),
+		array(
+			'public'            => false,
+			'hierarchical'      => false,
+			'labels'            => array(
+				'name'          => __( 'Themes' ),
+				'singular_name' => __( 'Theme' ),
+			),
+			'query_var'         => false,
+			'rewrite'           => false,
+			'show_ui'           => false,
+			'_builtin'          => true,
+			'show_in_nav_menus' => false,
+			'show_in_rest'      => false,
+		)
+	);
 }
 
 /**