type hint for to_json(indent=) should be Optional

[leifwalsh]

In #3071 and #3073 types were added to some to_json() methods (well, #3071 added the method entirely to TopLevelMixin) that specify indent as an int. In these, indent is only passed along to json.dumps, where it should be possible to pass None or a string as well: https://docs.python.org/3/library/json.html#json.dump

With indent typed as int and defaulting to 2 in the altair API, there is no way to get the compact indent=None serialization in a way that typechecks.

I think the types should be relaxed to str | int | None or something similar.

[binste]

Thanks for the detailed report @leifwalsh! Fully agree, I'll change it to Optional[Union[str, int]] for the next release.