Integrate with AbortSignal

[domenic]

Feedback from @annevk and @smaug---- is that you should be able to do something like this:

const controller = new AbortController();
const watcher = new CloseWatcher({ signal: controller.signal });

controller.abort();

which will work the same as

const watcher = new CloseWatcher();
watcher.destroy();

The advantage, of course, is for cases where you have a single AbortSignal tied to many different things, one of which might be a CloseWatcher.