feat: add typescript definition

[SimenB]

The v2 API is so simplified I think it shouldn't be a huge maintenance burden to keep the definition in this repo.

Types are based on the types in the README, plus the missing cloned event from #286. Constants were copied from the header file.

I'm unsure about the return type from watch. I based it on

fsevents/fsevents.js

Lines 26 to 30 in e468f15

	return () => {
	const result = instance ? Promise.resolve(instance).then(Native.stop) : null;
	instance = null;
	return result;
	};

, but I don't know what the promise resolves to? Also, should it always return a Promise regardless of instance === null or not? Happy to tweak that part of the code.

EDIT: I've opened up a separate PR with the change to always return a Promise, and you can choose to accept that or not. I can tweak the type in this PR based on that conclusion: #296

[paulmillr]

Thank you

[SimenB]

Thanks @paulmillr!