WP_Icons_Registry: Add filter icons_registry_core_manifest

This is so that the Gutenberg plugin can easily provide new core icons
in between major WordPress releases.

On the other hand, we will probably end up redefining whole methods on
the Gutenberg side as we improve WP_Icons_Registry, so it may be moot to
introduce a filter just for the manifest file.

Author: mcsf

src/wp-includes/class-wp-icons-registry.php

@@ -44,6 +44,7 @@ protected function __construct() {
 		$icons_directory = __DIR__ . '/icons/';
 		$icons_directory = trailingslashit( $icons_directory );
 		$manifest_path   = $icons_directory . 'manifest.php';
+		$manifest_path   = apply_filters( 'icons_registry_core_manifest', $manifest_path );
 
 		if ( ! is_readable( $manifest_path ) ) {
 			wp_trigger_error(