Implement User.state (v12)

[elle-j]

It should be possible to access the state (UserState) on a user. Implementation is missing for v12.

get state(): UserState {
  throw new Error("Not yet implemented");
}

Also, the UserState enum needs to be updated to align with this to:

enum UserState {
  /** Authenticated and available to communicate with services. */
  LoggedIn = "LoggedIn",
  /** Logged out, but ready to be logged in. */
  LoggedOut = "LoggedOut",
  /** Removed from the app entirely. */
  Removed = "Removed",
}

(Similar issue for v pre-12: Issue 5203)