[RFC] Move to Typescript

With more OS projects now planning to move to Typescript (jest, Vue.js, etc.), I think it would be time to think about doing the same. Typescript has matured and now with eslint and babel both supporting Typescript as well, it should be future proof.

Additionally, I think the tooling around Typescript (speed, autocompletion), etc. is far head of flow currently.

I would start with "rewriting" smaller packages (like `css-functions` and `theming`) in the beginning.
Also, I started writing the new docs with Typescript.

__Are you willing to implement it?__
Yes

EDIT: I started rewriting the core in my fork in Typescript as an experiment.