TypeScript type definitions

Dear all,

Some people (including myself) prefer using typed Javascript, using TypeScript for instance.

As I was not able to find any type definitions for the RDFJS interfaces,
I’ve implemented these myself here:
https://github.com/rubensworks/DefinitelyTyped/commit/44292d468be8893f589a1ee4a76dd8d50b588ce3

Before I make a PR with these types to the DefinitelyTyped repo,
so that these types would become available as `@types/rdf-js`,
I first want to make sure everyone here agrees with this implementation.

The JSDoc has been taken directly from the spec.

Any feedback or comments are welcome!

Kind regards,
Ruben Taelman