Move rest api loads into conditional that confirms in rest mode (#6649)

* Move rest api loads into conditional that confirms in rest mode

The rest api files should only be loaded if the site is in rest server mode.
I initially tested using
    `defined( 'REST_REQUEST') && REST_REQUEST`

however this wasn't detected and created errors, thus use of class_exists
checking for WP_REST_Controller

lib/load.php

@@ -9,20 +9,26 @@
 	die( 'Silence is golden.' );
 }
 
+// These files only need to be loaded if within a rest server instance
+// which this class will exist if that is the case.
+if ( class_exists( 'WP_REST_Controller' ) ) {
+	require dirname( __FILE__ ) . '/class-wp-rest-blocks-controller.php';
+	require dirname( __FILE__ ) . '/class-wp-rest-block-renderer-controller.php';
+	require dirname( __FILE__ ) . '/rest-api.php';
+}
+
 require dirname( __FILE__ ) . '/meta-box-partial-page.php';
 require dirname( __FILE__ ) . '/class-wp-block-type.php';
 require dirname( __FILE__ ) . '/class-wp-block-type-registry.php';
-require dirname( __FILE__ ) . '/class-wp-rest-blocks-controller.php';
-require dirname( __FILE__ ) . '/class-wp-rest-block-renderer-controller.php';
 require dirname( __FILE__ ) . '/blocks.php';
 require dirname( __FILE__ ) . '/client-assets.php';
 require dirname( __FILE__ ) . '/compat.php';
-require dirname( __FILE__ ) . '/rest-api.php';
 require dirname( __FILE__ ) . '/plugin-compat.php';
 require dirname( __FILE__ ) . '/i18n.php';
 require dirname( __FILE__ ) . '/parser.php';
 require dirname( __FILE__ ) . '/register.php';
 
+
 // Register server-side code for individual blocks.
 foreach ( glob( dirname( __FILE__ ) . '/../core-blocks/*/index.php' ) as $block_logic ) {
 	require $block_logic;