Isomorphic functions

[nick-potts]

It'd make sense for some things like getAuth to be isomorphic, so that it doesn't have to do a hop to the server every time:
https://tanstack.com/start/latest/docs/framework/react/guide/environment-functions#isomorphic-functions

[nicknisi]

I'm wondering if this would still be worthwhile with #17 merged? With this, you can provide an initialAuth property to AuthKitProvider to preload user info to display without the flash from the initial getAuth call.