Re: [whatwg/dom] Standardize certain Microsoft event extensions (#334)

Yeah, let's try to avoid any churn (I'm try to be careful here...).

My thinking here: 

`Event.returnValue`:

* If you set `returnValue` to `false`, prevent the default action.
* Otherwise, just set the value.

(I haven't tested existing handling of `0` or `null` yet, on my TODO list.)

> Gecko, and WebKit as of @cdumez's recent changes, implement that part of the HTML spec perfectly;

Maybe I misunderstand (odds are good...): https://miketaylr.com/bzla/beforeunload.html in WebKit shows the beforeunload prompt when `returnValue` is set to the empty string, which is not what HTML says. So currently the only maintained browser that has that behavior is Gecko. I suspect it's not needed.

`BeforeUnload.returnValue` at that point just behaves like `Event`, so I think we can get rid of it.

I'd prefer to not touch any other aspect of `beforeunload` event handling at all, if possible.


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