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);
}

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] All unit tests pass (melos run test:unit:all doesn't fail).
• [x] I read and followed the Flutter Style Guide.
• [x] I signed the CLA.
• [x] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.