RFC: New types for typings

[layershifter]

We have customPropTypes for our shorthands and it works fine 😄 But, their represent in typings is not obvious.

My suggestion is to add new types for customPropTypes and update existing typings:

type ShorthandCollection = Array<ShorthandItem>; // for customPropTypes.collectionShorthand
type ShorthandContent = React.ReactNode; // for customPropTypes.contentShorthand
type ShorthandItem = any; // for customPropTypes.itemShorthand

@levithomason any feedback?

[levithomason]

This sounds really sane. It will make updates much easier to these special types.

[layershifter]

I block this issue before we have the decision in #1364.