Update Composer config

Author: swissspidy

composer.json

@@ -25,7 +25,8 @@
     "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
   },
@@ -58,12 +59,14 @@
     "behat-rerun": "rerun-behat-tests",
     "lint": "run-linter-tests",
     "phpcs": "run-phpcs-tests",
+    "phpstan": "run-phpstan-tests",
     "phpcbf": "run-phpcbf-cleanup",
     "phpunit": "run-php-unit-tests",
     "prepare-tests": "install-package-tests",
     "test": [
       "@lint",
       "@phpcs",
+      "@phpstan",
       "@phpunit",
       "@behat"
     ]