webspeechapi: Fix event types in `SpeechSynthesisUtterance`

The DefinitelyTyped definitions for this API have some differences that appear correct. SpeechSynthesisUtterance.onstart and other methods should have a SpeechSynthesisEvent as their event type, and not just Event. We already have a SpeechSynthesisEvent type defined in lib.dom.d.ts, but aren't using it everywhere we should.

[saschanaz]

Fixed by #526