Support for json.Number in generated output

[shellhazard]

Hi there, thanks a lot for this project.

I see in the roadmap there's an option to use the json.Number type as output in the roadmap. This has a real practical use - it would allow unmarshalling JSON strings and JSON numbers into the same generated field without errors (see playground).

Was wondering if there was anything blocking the implementation of that functionality. If it's just a matter of it not being done yet and there's no wider structural reason I'd be happy to take a look myself.

Cheers.