Add isInitialized

[ralfstory]

This PR adds an isInitialized function which wraps window._fs_initialized. This is useful for code that could potentially repeatedly call init (like useEffects), leading to noisy console warnings. Checking for initialization before calling init would be helpful in this situation.

[patrick-fs]

@ralfstory would you also please include isIniatlized in the type definition (index.d.ts) and update the readme?

[luser]

I rebased your changes and squashed them while I was at it. I dropped the changes to package-lock.json since there were conflicts and they didn't seem relevant to the change at hand. I'll merge this now, thanks for the change!

[ralfstory]

@luser thanks for getting to this!! no longer needed it for what I was working on, so it was easy to fall through - still a helpful addition in any case!