Editor: add navigation fallback.

Creates a fallback menu for the Navigation block including an API endpoint to retrieve it.

Props get_dave, spacedmonkey, kebbet, flixos90, mikeschroder, ramonopoly, audrasjb.
Fixes 58557.


git-svn-id: https://develop.svn.wordpress.org/trunk@56052 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -377,6 +377,10 @@ function create_initial_rest_routes() {
 	// Site Editor Export.
 	$controller = new WP_REST_Edit_Site_Export_Controller();
 	$controller->register_routes();
+
+	// Navigation Fallback.
+	$controller = new WP_REST_Navigation_Fallback_Controller();
+	$controller->register_routes();
 }
 
 /**