[MODULES] Rename extensions to modules, add example plugin, change plugin location

plugins/Test/Controller/TestController.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Controller;
+
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\Response;
+
+class TestController extends AbstractController
+{
+    public function __invoke()
+    {
+        return new Response('<div style="background: #333; text: #999"> Test controller </div>');
+    }
+}