docs: add @var for better DX

Author: kenjis

system/Common.php

@@ -28,6 +28,7 @@
 use CodeIgniter\HTTP\RedirectResponse;
 use CodeIgniter\HTTP\RequestInterface;
 use CodeIgniter\HTTP\ResponseInterface;
+use CodeIgniter\Language\Language;
 use CodeIgniter\Model;
 use CodeIgniter\Session\Session;
 use CodeIgniter\Test\TestLogger;
@@ -732,6 +733,7 @@ function is_windows(?bool $mock = null): bool
      */
     function lang(string $line, array $args = [], ?string $locale = null)
     {
+        /** @var Language $language */
         $language = service('language');
 
         // Get active locale