[react-is] The major versions of react and react-is must be peerDependencies related.

(This is not directly related to React 18, but it is a discussion about breaking changes, so I'm proposing it as a topic for the next major release.) Clearly react-is works on the assumption that it will be used with a matching version of react. Therefore, **react should be listed as a peerDependencies of react-is.** I brought this up from a discussion involving a package that depends on react-is downstream.