add phpstan

Author: ylebre

composer.json

@@ -19,6 +19,7 @@
         "phpmailer/phpmailer": "^6.10"
     },
     "require-dev": {
-        "phpunit/phpunit": "^12.2"
+        "phpunit/phpunit": "^12.2",
+        "phpstan/phpstan": "^2.1"
     }
 }