fix: allow events to return a promise

[OmgImAlexis]

This allows .on() to accept a promise as it's callback. For example this is very common.

ws.on('close', async () => {
  await db.doThing();
});

This also changes the class to using typed-emitter which makes it a lot easier to type the events. Would be nice if they were fully typed at some point too instead of using args.

[OmgImAlexis]

Here are the types for the events if you also want to add that change.

type Events = {
    message: (message: string) => void | Promise<void>;
    close: (code: number, message: string) => void | Promise<void>;
    drain: () => void | Promise<void>;
    ping: () => void | Promise<void>;
    pong: () => void | Promise<void>;
}