feat: messages extension (#4028)

* feat: private messages

extensions/messages/tests/integration/setup.php

@@ -0,0 +1,12 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+$setup = require __DIR__.'/../../../../php-packages/testing/bootstrap/monorepo.php';
+
+$setup->run();