Editor: Add block theme infrastructure

Adds the required infrastructure to render block-based themes. This is sourced
from the Gutenberg plugin.

Fixes #54335.
Props bernhard-reiter, youknowriad, ntsekouras, hellofromtonya.


git-svn-id: https://develop.svn.wordpress.org/trunk@52062 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-block-template.php

@@ -100,4 +100,13 @@ class WP_Block_Template {
 	 * @var boolean
 	 */
 	public $has_theme_file;
+
+	/**
+	 * Whether a template is a custom template.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @var boolean
+	 */
+	public $is_custom = true;
 }