test: add Vitest unit tests for location-detector and country-flags

[Ghifariezra]

Fixes #232

Additionally, at the time of submission of this PR:

• The referred issue is not blocked currently
• All unittests passed after changes were made

Summary

Adds Vitest unit tests for detectCountry (location-detector.ts) and getCountryCode (country-flags.ts).

Coverage

• Exact matches (country names and major cities), case-insensitivity, whitespace and punctuation handling
• Word-boundary cases (e.g. Romania vs Oman, Somalia vs Mali, Nigeria vs Niger)
• Edge cases: null, empty, whitespace-only, special characters
• getCountryCode for valid slugs, unknown slugs, and slugs with an empty ISO code

Verification

• pnpm test: 24 files, 198 tests passed
• pnpm lint: no errors

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[github-actions]

Thank you for the pull request! ✅

A maintainer will review this soon. Please be patient while we take a look. 🙌

[O2sa]

🎊 Welcome, @Ghifariezra! Your first contribution has been merged! 🚀

Thank you for helping improve the project! If you find this tool useful, please consider giving us a ⭐ star on GitHub—it helps more developers find our work and motivates us to keep improving!

[Ghifariezra]

Thanks for the quick review and merge, @O2sa! Glad to contribute. ⭐ Starred the repo.