Provide TypeScript .d.ts type declarations

[tbazin]

Would you accept a PR providing TypeScript type declarations for the package? I have an initial version of those that I could contribute to the repo!
Alternately, I could add those to the @DefinitelyTyped /DefinitelyTyped repository.

[mscottnelson]

I would love a PR for this. Please follow the official guidance on bundling the type declarations.

I personally think it would be nice to someday refactor into TypeScript. Having it colocated here may help an eventual transition. Thank you!