Add focusable to view props (#56137)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56137

Changelog: [Internal]

Adds `focusable` as a prop to iOS so we can re-use it on views for `canBecomeFocused` in AppleTV apps. We do not intend to make this an actual public API change, but it is non-destructive.

Reviewed By: joevilches

Differential Revision: D96752037

fbshipit-source-id: 72a55cc3d6cc589eff49ae8a129a37920f000b94