Export CloseEvent, ErrorEvent, MessageEvent classes

[edolix]

I don't know if you guys like the idea to make these classes public but i ended up implementing the CloseEvent class in one of my project (since i had to trigger it manually).

I thought it would be useful for others so that's the reason for the PR. If you think otherwise, feel free to close it!

Thanks for the great work on this package 💯

[lpinca]

Thank you Edoardo but I think it is not a good idea to expose them because

1. We are not strictly following the EventTarget specification (Use Node.js 15 native EventTarget object #1818).
2. Those events should not be dispatched manually.

[edolix]

Sounds good @lpinca!
Thanks for the quick review - i'll close the PR!