chore: add cover composer script

Author: G360-Niek

.gitignore

@@ -1,2 +1,3 @@
+/coverage
 /vendor
 composer.lock

composer.json

@@ -30,6 +30,7 @@
         }
     },
     "scripts": {
-        "test": "vendor/bin/phpunit"
+        "test": "vendor/bin/phpunit",
+        "cover": "php -d xdebug.mode=coverage vendor/bin/phpunit --coverage-html=coverage"
     }
 }