Add detected language objects

What does this MR do and why?

Gitlab::LanguageDetection exposes detected language fields through name-based helper methods. This makes later language matching changes harder to express without passing names between helpers.

This MR:

• Adds Gitlab::LanguageDetection::DetectedLanguage.
• Exposes detected language values through #detected_languages.
• Keeps existing name-based helpers behavior-compatible.
• Adds specs for detected language objects and color lookup.

References

#602294 (closed)

How to set up and validate locally

1. Run the targeted specs:

   bin/rspec spec/lib/gitlab/language_detection_spec.rb

2. Run RuboCop:

   bundle exec rubocop lib/gitlab/language_detection.rb spec/lib/gitlab/language_detection_spec.rb

MR acceptance checklist

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