Event handling consistency between Awareness and LoroDoc

[djrenren]

I just rigged these up in my application and found these two types to be confusingly different.

// LoroDoc
const unsubscribe = doc.subscribe((update) => {});
unsubscribe();

// Awareness
const listener = (update, origin) => {};
awareness.addListener(listener);
awareness.removeListener(listener);

LoroDoc returns an unsubscribe function while Awareness uses an approximation of the EventTarget interface.
In my ideal world, they'd both just use EventTarget (available in both Node and Browser), but wasm_bindgen doesn't support that so we'd have to wrap LoroDoc which is not ideal.