Add PHPStan configuration for Symfony4 tests

- Created a new phpstan.neon file in the Symfony4 tests directory.
- Configured paths and excluded Symfony3 directory.
- Added bootstrap files for autoloading.
- Set dynamic constant names and adjusted reporting settings.
- Established PHPStan level to 6 for stricter analysis.

modules/empikmarketplace/src/Adapter/ConfigurationAdapter.php

@@ -27,6 +27,7 @@ class ConfigurationAdapter
     const CONF_SHIPPED_ORDER_STATE = 'EMPIK_SHIPPED_ORDER_STATE';
     const CONF_CARRIER_MAP = 'EMPIK_CARRIER_MAP';
     const CONF_ID_EMPLOYEE = 'EMPIK_ID_EMPLOYEE';
+    const CONF_DEFAULT_VAT_RATE = 'EMPIK_DEFAULT_VAT_RATE';
 
     const ENV_TEST = 'https://stg1.marketplace.empik.com';
     const ENV_PROD  = 'https://marketplace.empik.com';