feat(ui_auth): export localizedErrorText (#189)

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:

```dart
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);
}
```

Author: lesnitsky

packages/firebase_ui_auth/lib/firebase_ui_auth.dart

@@ -81,7 +81,7 @@ export 'src/widgets/email_form.dart'
 export 'src/widgets/email_input.dart';
 export 'src/widgets/email_link_sign_in_button.dart';
 export 'src/widgets/email_sign_up_dialog.dart';
-export 'src/widgets/error_text.dart' show ErrorText;
+export 'src/widgets/error_text.dart' show ErrorText, localizedErrorText;
 export 'src/widgets/forgot_password_button.dart';
 export 'src/widgets/internal/oauth_provider_button.dart'
     show OAuthProviderButton, OAuthButtonVariant;

packages/firebase_ui_auth/lib/src/widgets/error_text.dart

@@ -43,11 +43,13 @@ class ErrorText extends StatelessWidget {
   ///
   /// Example usage:
   /// ```dart
-  /// ErrorText.localizeError = (BuildContext context, fba.FirebaseAuthException e) {
+  /// ErrorText.localizeError = (BuildContext context, FirebaseAuthException e) {
+  ///   final defaultLabels = FirebaseUILocalizations.labelsOf(context);
+  ///
   ///   return switch (e.code) {
   ///     'user-not-found' => 'Please create an account first.',
   ///     'credential-already-in-use' => 'This email is already in use.',
-  ///     _ => 'Oh no! Something went wrong.'
+  ///     _ => localizedErrorText(e.code, defaultLabels) ?? 'Oh no! Something went wrong.',
   ///   }
   /// }
   static String Function(