Support a mouse-up JavaScript action on form fields (onClick option) #1792

Following the invitation in `docs/forms.md`'s Advanced Form Field Use section: "If an option is not supported, open an issue on Github and it will be considered for addition to the API."

The gap

There's no supported way to attach a mouse-up JavaScript action to a form field — most usefully, a push button that runs custom logic when clicked. The only code path that ever reaches `AA` is `mapFormat()`, and only together with a `format` option, which is meant for keystroke/format validation, not arbitrary actions. Any other use of `AA` is silently dropped with no error.

This came up building an interactive PDF with clickable push buttons that toggle state via a mouse-up action.

Proposed addition

An `onClick` option, accepted by all form annotation methods, mapping to the field's `AA.U` mouse-up action — accepting either a plain string or a function (stringified into the action, called with Acrobat's own `app`/`getField`/`display`/`event` as arguments and `this` bound to the Document).

I have a working implementation with tests, docs, and a small opt-in TypeScript types file for the handful of Acrobat globals the option's function form needs — see #1790.