feat(web): Allow setKeyboardForControl with IFrames

[darcywong00]

Reference: https://stackoverflow.com/questions/69694488/keyman-setdefaultkeyboardforcontrolpelem-keyboard-languagecode-function-is

A user asked asked how to get setDefaultKeyboardForControl to work with an iframe element.

It's currently a system limitation that @mcdurdin says

Seems like a limitation we could fix...

[jahorton]

As of #7343, we're one step closer to this.

That said, one notable issue with setKeyboardForControl with iframe elements - especially non-design-mode ones - is that they may contain multiple elements. It is technically possible, I believe, to call setKeyboardForControl on each of those elements.

This then raises the question: what should setKeyboardForControl actually do in this scenario? Should it iterate over all of the iframe's internal elements and use setKeyboardForControl on each? This seems the simplest solution.

[mcdurdin]

setKeyboardForControl should only be used on iframes which are in designmode=true. It's intended for use for a single edit element, whether that is an input, textarea, div+contentEditable, or iframe+designMode.

We probably should even test for that and throw if caller attempts to setKeyboardForControl for any non-design-mode iframe.