Add initial TypeScript type declarations

[tbazin]

Hi!

Here's a first proposition for bundled TS type declarations.
It's probably not perfectly clean, I wasn't sure what to do with private class methods for instance, and ultimately I believe the library could be rewritten in native TypeScript, alleviating the need for such a decoupled approach, but that should already be quite a usable basis!

[mscottnelson]

@tbazin thank you for your work on this! It seems maintenance has stalled. Feel free to keep updating this PR. Since review is taking awhile, another alternative to sharing these would be to send them to DefinitelyTyped. Perhaps while awaiting review, you might consider pushing these types to DefinitelyTyped so that others may more easily benefit from your work.

Hopefully NexusUI will see native types and more regular maintenance at some point, but until then, you’ll probably have an easier time getting your contributions merged there.