Add Type Definitions

[jpike88]

See title.

@types/async definitions are way out of date, would be better for everyone if you guys integrated them into your main project

[Troublor]

Agree, it's annoying when encounter outdated type definitions.

[justarandomgeek]

Indeed, it fairly effectively prevents any reasonable usage of newer features from TypeScript...

[aearly]

I've considered a re-write in TS (or at least adding Typedoc comments to everything and generating a .d.ts automatically). It would be a lot of work though.

[mnpenner]

Fwiw there are tools to convert JS to TS. It'll add "any" to any types it's not sure about but it's a bit quicker than doing by hand.