add extension

Author: freekmurze

composer.json

@@ -22,10 +22,11 @@
     "require": {
         "php": "^7.4",
         "ext-dom": "*",
+        "ext-json": "*",
         "phpunit/phpunit": "^8.3|^9.0",
         "symfony/property-access": "^4.0|^5.0",
         "symfony/serializer": "^4.0|^5.0",
-        "symfony/yaml": "^4.0|^5.0"
+        "symfony/yaml": "^4.0|^5.0",
     },
     "autoload": {
         "psr-4": {