[Tooling] Fix React Native types

[javamonn]

As noted here, palette is currently typed with DOM types regardless of the runtime platform.

Emission currently type-checks, but as a result, TypeScript would erroneously allow a DOM-only prop to be passed without error.

There was some initial exploration work towards fixing this in https://github.com/javamonn/palette/tree/upgrade-typings, though I'm not sure it's the best way forward - it seems like we might need separate stsconfig`s and separate builds.

[javamonn]

An instance in Emission where this caused a type error we had to workaround with a cast: https://github.com/artsy/emission/pull/1223/files#diff-ec3ba1930b2ad578adef27656b4b5cacR18