Make AppState type more accessible

[palfrey]

Describe the problem you'd like to have solved

The onRedirectCallback of Auth0Provider takes a function that has an argument of AppState. However, the type of AppState isn't easily available outside the library, which makes TypeScript work without using any harder.

Describe the ideal solution

import { AppState } from '@auth0/auth0-react' should just work, via exporting AppState.

Alternatives and current workarounds

import { AppState } from '@auth0/auth0-react/dist/auth0-provider' works, but is annoying.

[adamjmcgrath]

Good point... thanks @palfrey