Provide list of supported locales when lazy loading

### Clear and concise description of the problem

When eagerly loading the locales a list of supported locales is provided by `useI18n().availableLocales`.

There is no such list when lazy loading the locales since `availableLocales` only includes loaded locales.

### Suggested solution

In https://github.com/lukashass/woodpecker/blob/1bfb9a322f9f8b94ec223afc7eb9085d54f3a16b/web/vite.config.ts#L34-L53 I used a vite virtual module that provides a list of supported locales derived from the filenames in the vueI18n include path.

I think this might be a good feature to be included in the vite plugin (or unplugin?)

### Alternative

A manual list of supported locales is of course possible, but not really desirable IMHO.

### Additional context

_No response_

### Validations

- [X] Read the [Contributing Guidelines](https://github.com/intlify/bundle-tools/blob/main/.github/contributing.md).
- [X] Read the README
- [X] Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.