Move to Typescript

[lettertwo]

We've got a fair amount of Flow in these packages, but i think the writing's on the wall WRT who will win the typed JS war.

Some links that might help:

• TypeScript With Babel: A Beautiful Marriage
• Incremental Migration to TypeScript on a Flowtype codebase
• flow-to-typescript - This might be useful if it works. In particular, it seems this PR might eventually deliver.
• Maybe we can lift from CRA's typescript configs

[aamorozov]

@lettertwo did it all 🎉