Update the Steps to fire beforeunload to allow to be skipped by WebDriver-BiDi

[Lightning00Blade]

What problem are you trying to solve?

Currently the WebDriver-BiDi spec browser.close command -
Defines step 4.3 - Close any top-level browsing contexts without prompting to unload.
This will be used also for another command - browsingContext.close when w3c/webdriver-bidi#518 is merged.

We want to close a top-level browsing context without prompting any beforeunload handles.
In the HTML spec there is no definition of how this has to be done and currently works by "magic".

What solutions exist today?

No response

How would you solve it?

A suggestion on how to implement this can be found on this comment w3c/webdriver-bidi#518 (comment) :

It seems like we could reuse the existing `WebDriver BiDi user prompt opened` steps 
by allowing them to return a value that specifies whether to show the prompt or not...

Anything else?

No response