Switch react-navigation types to be generated from upstream

[gnprice]

This will then greatly simplify handling type definitions in attempting to take the next major React Navigation upgrade, #4936.

I plan to do this using TsFlower: https://www.npmjs.com/package/tsflower

We'll still need some small ad-hoc fixes to the type definitions, but they should be vastly smaller than the type definitions as a whole.