Add phpstan as dev dep

Author: oschwald

composer.json

@@ -21,7 +21,8 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "*",
         "phpunit/phpunit": "^8.0 || ^9.0",
-        "squizlabs/php_codesniffer": "3.*"
+        "squizlabs/php_codesniffer": "3.*",
+        "phpstan/phpstan": "*"
     },
     "autoload": {
         "psr-4": {