Add TypeScript types

[erqiu-sj]

Is there a typescript version?

[camwiegert]

There are currently no types for this package. I've considered writing them but haven't found time yet.

[erqiu-sj]

Maybe if I have time, I will write a set based on this

[camwiegert]

I think the best way to provide types for this would be to use Typescript JSDoc annotations. That way we don't need to introduce a build step.