Add Behat

Add Behat as a dev dependency and add the example `ls` example.

Due to a deprecation error, I needed to downgrade to PHP 8.1 on stream,
though this would error the main `app:generate` command as I'm using
`readonly` classes that were introduced in PHP 8.2.

src/DataTransferObject/TemplateFile.php

@@ -4,12 +4,12 @@ declare(strict_types=1);
 
 namespace App\DataTransferObject;
 
-readonly final class TemplateFile
+final class TemplateFile
 {
     public function __construct(
-        public string $data,
-        public string $name,
-        public string|null $path = null,
+        readonly public string $data,
+        readonly public string $name,
+        readonly public string|null $path = null,
     ) {
     }
 }