fix: case-insensitive extension validation and robust language detection (fixes #685)

[ronits2407]

This PR addresses issues with file extension validation and language detection, particularly on case-insensitive file systems like Windows.

Fixes #685

Changes:

• Case-Insensitive Validation: Updated isValidLanguage and getLanguage to normalize extensions to lowercase before checking. This fixes issues where files like f.CPP were incorrectly flagged as unsupported.
• Improved Language Detection: Refactored getLanguage to correctly handle extensions like .cc and .cxx by checking both language keys and supported extension values in the config.
• Enhanced Error Feedback: Improved checkUnsupported to provide a specific "No active editor found" message when no file is active, instead of the misleading "Unsupported file extension" error.
• Localization: Added translations for the new error message in English and Simplified Chinese.
• Code Style: Fixed formatting issues to comply with the project's Prettier and ESLint rules.

These changes improve the user experience and prevent confusing errors during file renaming or creation.

[agrawal-d]

Thanks!