Expose the constructor for CheckedFromJsonException

[kevmoo]

A bit scary, but it turns out that's it's really useful.

Better than having folks call $checkedNew to get the same behavior.

https://github.com/dart-lang/json_serializable/blob/f8dd6b895e8b8e09712b50988bee209976747675/json_annotation/lib/src/checked_helpers.dart#L47