Infra: Automatically verify index.d.ts

[ryan-roemer]

No idea if this is possible, but if we could, do some type of introspection on our flow code (probably generated lib or es) and break the build if something is totally incompatible with our TS type defs.

[stereobooster]

There is https://github.com/bcherny/flow-to-typescript