Enable SSR support in `useStore` without breaking non-SSR usage

The fix for SSR usage of `useStore` in https://github.com/nanostores/preact/pull/10 may introduce breaking changes for all other use-cases. This PR changes the approach, requiring the user to explicitly opt in to the SSR feature so that other use-cases will not be affected. This is necessary because Preact does not have the kind of low-level support for handling server vs client rendering provided by React's [`useSyncExternalStore`](https://react.dev/reference/react/useSyncExternalStore)