Extend HTMLView to work as dialog box returning data

Following recent Discord discussion we realise we could use HTMLViews for more advanced dialog boxes. (They probably won't be performant compared to native controls, so this isn't a long term solution.)

They can already be used to call plugin commands, as prototyped by @dwertheimer and @EduardMe. This request is to allow return value(s) to be passed back to the calling JS, so it can act like a await CommandBar.something() call.

Further, @dwertheimer also adds that "can you please add a parameter called shouldDimMainWindow:boolean = true so I can override the dimming layer if I want?"

So we want to be able to say something like:

const [param, color] = await HTMLView.showModal(html, 'Window title', 500, 500, false)