Update Composer config

Author: swissspidy

composer.json

@@ -3,7 +3,8 @@
     "description": "Source files for the handbook that is generated for make.wordpress.org.",
     "config": {
         "allow-plugins": {
-            "dealerdirect/phpcodesniffer-composer-installer": true
+            "dealerdirect/phpcodesniffer-composer-installer": true,
+            "phpstan/extension-installer": true
         },
         "sort-packages": true
     },
@@ -24,11 +25,13 @@
         "lint": "run-linter-tests",
         "phpcs": "run-phpcs-tests",
         "phpcbf": "run-phpcbf-cleanup",
+        "phpstan": "run-phpstan-tests",
         "phpunit": "run-php-unit-tests",
         "prepare-tests": "install-package-tests",
         "test": [
             "@lint",
             "@phpcs",
+            "@phpstan",
             "@phpunit",
             "@behat"
         ]