[fix] ⚡️ Use RO singleton instead of iframe if supported

[TheCymaera]

Uses ResizeObserver singleton for add_resize_listener instead of iframes if it is supported by the runtime.

Fixes #7583
Fixes #4233

[Conduitry]

I forget where this was mentioned, but we do need these to be separate bindings, because the ResizeObserver is measuring something slightly different from the existing iframe mechanism, and we don't want people's results to change if they suddenly start using ResizeObserver.

[dummdidumm]

Also see #5963 which is already an attempt at implementing this, but stalled due to difficulties around the initial size. I you want you can pick up the work in #5963 instead.

[TheCymaera]

Found the comment: #5524 (comment)
What if I put it behind a compiler option?

resizeDetection: "iframe"|"resizeObserver"|"auto"

If so, what should the default be?
Edit: Maybe should use "iframe" for default and decide again for 4.0.