Update Vietnam holidays: add th localization

[PPsyrius]

Proposed change

Adds Thai (th) localization for Vietnam holidays.

Note

As there's no single official translation for Vietnamese Lunar New Year in Thai usage from the government (The Thai embassy in Ho Chi Minh city used English for their holiday's notice), I ended up using วันตรุษเต๊ต as used by the Voice of Vietnam and the Thai-Vietnamese Friendship Association over วันตรุษเวียตนาม and วันตรุษญวน.

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency update (version deprecation/pin/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've run make pre-commit, it didn't generate any changes
• I've run make test, all tests passed locally

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (bb25790) to head (6b2dc0d).

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #2060   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          190       190           
  Lines        11539     11539           
  Branches      1743      1743           
=========================================
  Hits         11539     11539           

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