REST API: Support ordering response collection by listed slugs.

Adds an "include_slug" orderby value for REST API collections to permit returning a collection filtered by slugs in the same order in which those slugs are specified.
Previously, the order of slugs provided with the ?slug query parameter had no effect on the order of the returned records.

Props wonderboymusic, ocean90, boonebgorges.
Fixes #40826.



git-svn-id: https://develop.svn.wordpress.org/trunk@41760 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -874,9 +874,10 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 		// Map to proper WP_Query orderby param.
 		if ( isset( $query_args['orderby'] ) && isset( $request['orderby'] ) ) {
 			$orderby_mappings = array(
-				'id'      => 'ID',
-				'include' => 'post__in',
-				'slug'    => 'post_name',
+				'id'            => 'ID',
+				'include'       => 'post__in',
+				'slug'          => 'post_name',
+				'include_slugs' => 'post_name__in',
 			);
 
 			if ( isset( $orderby_mappings[ $request['orderby'] ] ) ) {
@@ -2109,6 +2110,7 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 				'parent',
 				'relevance',
 				'slug',
+				'include_slugs',
 				'title',
 			),
 		);