Add beforeunload

[autonome]

Hrm, I just saw the deprecated keys conversation in chat, and it seems like this feature should've given a warning or failure? Maybe I didn't see it, if just a warning.

Here we've got a non-deprecated interface where the implementation key is deprecated.

MDN doesn't mention deprecation, just recommends using other approaches.

Caniuse highlights the deprecation in the window implementation, but not for the interface.

Ok, just a sub-key is deprecated. Neither the interface or implementation are.

Will include it, commented out, with an explanation.

Edit again again:

• added 4 subkeys from BCD
• 3 are deprecated
• 1 is not, but makes the feature baseline false b/c Safari iOS

And I think that's seems right. Using this feature in the modern way is not baseline at this time.

Having a better explicit way to park deprecations would be good to figure out.

[jamesnw]

Using this feature in the modern way is not baseline at this time.

I agree with this assessment. This might be a good candidate for Interop... 🤔

[autonome]

Thanks for review @jamesnw. Merging, but also poking @ddbeck for his thoughts on how this handled the deprecated keys. Having them in-place adjacent to their fellow keys is good for visibility for future WF contributors. I'd prefer something explicit that allows programmatic checks, eg a new YAML top level deprecated section or something like that. But seems like a comment is good enough for now.