Add cs fixer and first controller with route /.

src/Controller/MainController.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Controller;
+
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\Routing\Attribute\Route;
+
+class MainController
+{
+    #[Route('/', name: 'main')]
+    public function homepage(): Response
+    {
+        return new Response('Hello World!');
+    }
+}