Consider spliting out MessageEvent

[rnons]

MessageEvent is an event type shared by many APIs. In my use case, I'm using Cross-document messaging, I don't want to import purescript-web-socket just to use the MessageEvent type.

[gbagan]

I'm implementing a binding for Web Workers and I'm facing this issue.
The functions "onmessage" and "onmessageerror" of Web Workers use the MessageEvent type.

https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent
https://developer.mozilla.org/en-US/docs/Web/API/Worker/message_event