User prompt handler capabilities (#1791)

Rewrite the user prompt handler configuration to allow overrides

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.

We also allow the possibility of multiple user prompts

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.

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Co-authored-by: Henrik Skupin <mail@hskupin.info>