Fix i18n collection name generation & path solution

[Muska-Ami]

This PR fixed i18n collections generation and path solution.

1. when locale is xx-XX (like zh-CN pt-BR), replace collection name to xx_XX (zh_CN)
2. when call queryCollection, lowercase xx-XX (zh_CN to zh-cn) to ensure that path is resolved properly.
3. fix when i18n is enabled, you can't create /lang/slug path due router will resolve it as /lang/lang?, but you still can't routing your doc to /lang/lang (e.g.: /en/en will cause a 404 Page Not Found result)
4. Fix a bug in Fix i18n collection name strategy #1246, which case you couldn't use zh-CN or pt-BR language code in configuration.

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commit: 7e23df2

[Muska-Ami]

Details

I've found a strange behavior here: when you set collection name to `docs_zh_CN`, but structure files like this:

- content
  - zh-CN
    - ...

Nuxt Content will find docs_zh-CN instead docs_zh_CN here 🤔.

Fixed.

[Muska-Ami]

This PR fixed i18n collections generation and path solution.

1. when locale is xx-XX (like zh-CN pt-BR), replace collection name to xx_XX (zh_CN)
2. when call queryCollection, lowercase xx-XX (zh_CN to zh-cn) to ensure that path is resolved properly.
3. fix when i18n is enabled, you can't create /lang/slug path due router will resolve it as /lang/lang?, but you still can't routing your doc to /lang/lang (e.g.: /en/en will cause a 404 Page Not Found result)