Refactor Lao New Year holidays

[PPsyrius]

Proposed change

Refractor existing Lao New Year holidays logic to better match the official calendars, move the in-lieu observance start from 2018 to 2012 to better match the actual situation.

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 python-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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6ec8d56) to head (0eabd6b).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #1962   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          188       188           
  Lines        11419     11422    +3     
  Branches      1795      1799    +4     
=========================================
+ Hits         11419     11422    +3     

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[sonarqubecloud]

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

[arkid15r]

Looks good, thank you @PPsyrius!