Support React 16/17 (possibly with seperate package)

Lots of companies (such as Microsoft) are slower to migrate to React 18. We can't use nanostores right now because React 17 isn't supported. It would be great if the package supported React 17, possibly using the [useSyncExternalStore shim](https://www.npmjs.com/package/use-sync-external-store). Since this would affect package size, it could be published as a separate package like `@nanostores/react-17`. The shim supports React 16 as well!