select[multiple] and/or a checkbox representation

[toastal]

Like radios and select for OR, there should be the AND option for an unspecified list of a thing that is translated to a select[multiple] -- and probably a list of input[type=checkbox] since most users find select[multiple] to be unintuitive to hold ctrl/option (although it's debatably a better UX on mobile to get a native pop-up with checkbboxes).

[hecrj]

I would like to finish #7 before thinking about what to add to Form next.

In any case, I plan on focusing on extensibility soon (i.e. making custom fields easier to create), which should indirectly help fix this issue.

In the meantime, I think the behavior you describe can be achieved folding over multiple Form.checkboxField.

That said, if anyone is up to implementing this, PRs are welcome!