MDL-82565 core: Add support for routing of front-end pages

This is the beginning of the end for non-routed pages in Moodle and the
start of SEO-friendly page URLs.

A 'shim' is provided for backwards compatibility.

lib/classes/router/route_loader_interface.php

@@ -31,6 +31,12 @@ interface route_loader_interface {
     /** @var string The route path prefix to use for API calls */
     public const ROUTE_GROUP_API = '/api/rest/v2';
 
+    /** @var string The route path prefix to use for API calls */
+    public const ROUTE_GROUP_SHIM = 'shim';
+
+    /** @var string The route path prefix to use for API calls */
+    public const ROUTE_GROUP_PAGE = '/';
+
     /**
      * Configure all routes for the Application.
      *