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/template.php

@@ -63,6 +63,8 @@ function get_query_template( $type, $templates = array() ) {
 
 	$template = locate_template( $templates );
 
+	$template = locate_block_template( $template, $type, $templates );
+
 	/**
 	 * Filters the path of the queried template by type.
 	 *