Editor: Update preloaded paths for the site editor

Add the `OPTIONS /wp/v2/pages` route to the list. The request is made via the `useBlockEditorSettings` hook at the top of the editor React tree. Preloading should improve the editor's loading time.

Avoid hardcoding paths for the 'attachment' post type; use the `rest_get_route_for_post_type_items` helper method instead.

Props mamaduka, noisysocks.
See #61884.

git-svn-id: https://develop.svn.wordpress.org/trunk@58984 602fd350-edb4-49c9-b593-d223f7449a82

Author: Mamaduka

src/wp-admin/site-editor.php

@@ -88,7 +88,8 @@ static function ( $classes ) {
 );
 
 $preload_paths = array(
-	array( '/wp/v2/media', 'OPTIONS' ),
+	array( rest_get_route_for_post_type_items( 'attachment' ), 'OPTIONS' ),
+	array( rest_get_route_for_post_type_items( 'page' ), 'OPTIONS' ),
 	'/wp/v2/types?context=view',
 	'/wp/v2/types/wp_template?context=edit',
 	'/wp/v2/types/wp_template_part?context=edit',