Add react-copy-write example

[React Copy Write](https://github.com/drcmda/react-contextual) is an immutable React state management library. Works with [Immer](https://github.com/mweststrate/immer).

See the [Demo](https://codesandbox.io/s/yp34vpk50j)

Review the repository's existing React state-management examples first; the issue does not name a target file or test. Use the linked React Copy Write demo and library description to determine the example's expected behavior, then verify the new example works consistently with the surrounding examples.

Written by the indexing model from the issue text.