REST API: Include permalink_structure in the settings route.

There are situations where the REST API client wants to be able to read and set the permalink_structure option. Though this doesn’t fix the use case of front-end rendered unauthenticated websites, it still has utility as a setting in it’s own right.

Props aduth.
See #41014.

Built from https://develop.svn.wordpress.org/trunk@42359


git-svn-id: http://core.svn.wordpress.org/trunk@42188 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/option.php

@@ -2020,6 +2020,11 @@ function register_initial_settings() {
 		)
 	);
 
+	register_setting( 'permalink', 'permalink_structure', array(
+		'show_in_rest' => true,
+		'type'         => 'string',
+		'description'  => __( 'Custom URL structure for permalinks and archives.' ),
+	) );
 }
 
 /**