Update Composer config

[dependabot skip]

Author: swissspidy

composer.json

@@ -27,7 +27,8 @@
         "sort-packages": true,
         "allow-plugins": {
             "dealerdirect/phpcodesniffer-composer-installer": true,
-            "johnpbloch/wordpress-core-installer": true
+            "johnpbloch/wordpress-core-installer": true,
+            "phpstan/extension-installer": true
         },
         "lock": false
     },
@@ -232,10 +233,12 @@
         "phpcs": "run-phpcs-tests",
         "phpunit": "run-php-unit-tests",
         "phpcbf": "run-phpcbf-cleanup",
+        "phpstan": "run-phpstan-tests",
         "prepare-tests": "install-package-tests",
         "test": [
             "@lint",
             "@phpcs",
+            "@phpstan",
             "@phpunit",
             "@behat"
         ]