Add Webapi.Dom.BeforeUnloadEvent.returnValue setter

[MoOx]

returnValue is writable : https://developer.mozilla.org/en-US/docs/Web/API/BeforeUnloadEvent

[TheSpyder]

Oh dear I forgot to check it compiled. I need to add branch CI and more tests 😂

This method conflicts with returnValue right above it, it needs a different name. I'll rename it before publishing.