Add JSON types to the type system as builtins

If you have a property of type QList<QJsonObject>, qmllint will complain about not knowing QJsonObject.

The engine can however handle JSON, and will happily convert it into JS objects. So qmllint shouldn't warn there. We need to teach qmllint to handle QJsonObject the same as it would handle QVariant (same for QJsonArray and QVariantList).