Add magento2-base as dev dependency for PhpStan

Otherwise, classes like Magento\TestFramework\Helper\Bootstrap are unknown

Author: schmengler

composer.json

@@ -41,6 +41,7 @@
     },
     "require-dev": {
         "roave/security-advisories": "dev-master",
+        "magento/magento2-base": "^2.4.0",
         "phpunit/phpunit": "^6.0|^9.0",
         "pds/skeleton": "^1.0",
         "phpro/grumphp": "^v0.21.0",