Implement `EventTarget` and `Event` builtin

[andreiltd]

This patch sets the stage for incoming AbortController and AbortSignal:

• implements Event interface: https://dom.spec.whatwg.org/#interface-event,
• implements CustomEvent interface: https://dom.spec.whatwg.org/#interface-customevent,
• implement EventTarget interface: https://dom.spec.whatwg.org/#interface-eventtarget,
• adds a global EventTarget instance,
• makes FetchEvent a subclass of Event and uses global EventTarget.

Closes: https://github.com/bytecodealliance/StarlingMonkey/issues/156

[guybedford]

Great to see this. Strictly speaking FetchEvent is supposed to be a subclass of this Event. Not sure how practical that will be though, but I think that's the WinterTC expectation.

[andreiltd]

Great to see this. Strictly speaking FetchEvent is supposed to be a subclass of this Event. Not sure how practical that will be though, but I think that's the WinterTC expectation.

Yeah, I think it should be doable similar to how we made File extending Blob. I will look into this when I have a moment. I think the other nice to have event would be CustomEvent.

[andreiltd]

Strictly speaking FetchEvent is supposed to be a subclass of this Event.

So this is now working as expected, for instance this code shows FetchEvent using Event interface (event.type).

async function handler(event) {
  console.log(event.type);
}

addEventListener("fetch", (event) =>
  event.respondWith(
    (async () => {
      await handler(event);
      return new Response(`Success`);
    })(),
  ),
);

[tschneidereit]

Looks great! Feel free to merge whenever