Accept 2-letter locales in localizedAttributes

[tacman]

From meilisearch/meilisearch#4904 (comment)

{
  "localizedAttributes": [
      {"locales": ["es"], "attributePatterns": ["*.es"]},
      {"locales": ["en"], "attributePatterns": ["*.en"]},
      {"locales": [], "attributePatterns": ["*"]}
  ]
}

It would be a lot easier if you also supported 2-character ISO codes (https://en.wikipedia.org/wiki/List_of_ISO_639_language_codes) instead of just the 3-character.

The reason is that many web developers code around the request locale, and using the 3-letter code isn't very standard. So I find myself trying to find a library that makes the mapping.

That is, I'd like

{
  "localizedAttributes": [
      {"locales": ["es"], "attributePatterns": ["*_es"]},
      {"locales": ["en"], "attributePatterns": ["*_en"]},
      {"locales": [], "attributePatterns": ["*"]}
  ]
}

[dureuill]

Implemented by meilisearch/meilisearch#4937