[WIP] Support React 18

[jhuleatt]

React 18 is now in beta.

This PR will investigate how we can/should use the new APIs available in React 18:

useSyncExternalStore

This can probably replace the bulk of the useEffect/useReducer stuff in useObservable 🎉

useId

Maybe this can help with caching?

useInsertionEffect

From the working group discussion:

The use case is specifically for inserting global DOM nodes like <style> or SVG . (Maybe global Portals.) It's not really meant to be used by anything else other than these CSS libraries.

Maybe useful for react-firebaseui?

[jhuleatt]

Note: BehaviorSubject now has a synchronous getValue method: https://rxjs.dev/api/index/class/BehaviorSubject#getvalue

[jhuleatt]

implemented in #583