The idea is that for BiDi we'll accept objects like:

{
  "alert": "dismiss",
  "beforeUnload": "ignore"
}

to allow different handling per prompt type, whilst falling back to
the appropiate defaults to maintain the current behaviour for
implementations that don't support BiDi.

Co-authored-by: Henrik Skupin <[email protected]>

The specifications allow one prompt per event loop, so instead of
assuming a single current user prompt, allow getting the user
prompt for the active browsing context's event loop.