feat: add Vietnamese translation (#548)

* Create vi.ts

Create i18n Vietnamese translation file.

* Update translation.ts

Added Vietnamese language support.

* Update config.ts

Add Vietnamese (vi) to be a possible value for the `lang` key.

* Create README.vi.md

Created Vietnamese version of README.

* Update README.md

Added navigation/link to Vietnamese version of README.

* Update README.md

fixed a mistake

README.md

@@ -15,7 +15,8 @@ A static blog template built with [Astro](https://astro.build).
 [**日本語**](https://github.com/saicaca/fuwari/blob/main/docs/README.ja.md) /
 [**한국어**](https://github.com/saicaca/fuwari/blob/main/docs/README.ko.md) /
 [**Español**](https://github.com/saicaca/fuwari/blob/main/docs/README.es.md) /
-[**ไทย**](https://github.com/saicaca/fuwari/blob/main/docs/README.th.md) (Provided by the community and may not always be up-to-date)
+[**ไทย**](https://github.com/saicaca/fuwari/blob/main/docs/README.th.md) /
+[**Tiếng Việt**](https://github.com/saicaca/fuwari/blob/main/docs/README.vi.md) (Provided by the community and may not always be up-to-date)
 
 ## ✨ Features