fix(composer): add ext-json as requirement

Author: Dominik Jansen

composer.json

@@ -27,6 +27,7 @@
         "php": "^7.4",
         "ext-curl": "*",
         "ext-dom": "*",
+        "ext-json": "*",
         "ext-mbstring": "*",
         "composer/ca-bundle": "^1.0",
         "oscarotero/html-parser": "^0.1.4",