Allow transformers to specify their style type

[arthur-fontaine]

React Native and React does not have the exact same style properties. We want a way to style in a safe way.

Let's say the type for styles in React Native is

type RNStyle = { backgroundColor: string; border: string; borderRadius: number }; // for the example, we suppose that in RN, we can only style backgroundColor, border and borderRadius

And in React

type RStyle = { border: string; borderRadius: string }

The resulting typed will intersect all the styles exported by the transformers:

type Style = { border: string }; // (backgroundColor is not in RStyle, and RN's borderRadius an R's borderRadius are incompatible