Add typescript's definition

[johannchopin]

Hey team 👋

Thanks a lot for the work put on this project that I really enjoy. I would enjoy it even more if there was official TypeScript type definition available. Since rollup is already used, it will be straightforward to add the types directly to the repo https://github.com/rollup/plugins/tree/master/packages/typescript. Another approach would be to add a .d.ts file to DefinitelyTyped.

I can take care of it if you want 👍

What do you think of this idea?