feat: Add Vietnamese localization

[chuyentt]

Description

This PR adds Vietnamese localization to the firebase_ui_localizations package. It includes translations for various labels, error messages, and button texts to support Vietnamese language users. This enhancement aims to improve the user experience for Vietnamese-speaking users, making the Firebase UI components more accessible and user-friendly in the Vietnamese context.

Related Issues

This PR addresses the need for additional language support in the FirebaseUI-Flutter package, specifically adding support for the Vietnamese language. It does not directly resolve any open issues but contributes to the broader goal of internationalizing the FirebaseUI-Flutter package.

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[russellwheatley]

@chuyentt - thanks for the PR 😄 . Could you ask another developer fluent in Vietnamese to double check the translations, please? And put an approval on the PR if they are correct 👍