Add type definitions

This is just an issue to track interest in type definitions for the framework.

If there is enough interest I will consider working on adding definitions as I have time. With the consent of the maintainers, this can be directly in the repo or if a separation is more desirable they can just be added to the Definitely Typed library.

https://github.com/DefinitelyTyped/DefinitelyTyped

Please let me know if this interests you by giving the thumbs up here.

I would also love some feedback from @mikermcneil