chore: add phpstan

Author: tinect

composer.json

@@ -16,6 +16,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "@stable",
+        "phpstan/phpstan": "@stable",
         "league/flysystem-adapter-test-utilities": "^3",
         "league/flysystem-memory": "^3.0",
         "league/flysystem-path-prefixing": "^3.3",