feat(ui_auth): export localizedErrorText

[lesnitsky]

When ErrorText.localizeError is used, it becomes a single source of truth. This means all error messages should be localized by developer. A subset of errors is already localized by Firebase UI, so localizedErrorText that is used internally is now public and could be used inside ErrorText.localizeError.

Example:

ErrorText.localizeError = (BuildContext context, FirebaseAuthException e) {
  if (e.code == "user-not-found" || e.code == "wrong-password") return "Wrong credentials";

  final defaultLabels = FirebaseUILocalizations.labelsOf(context);
  return localizedErrorText(e.code, defaultLabels);
}

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