Update Chile holidays: add bank holidays

[KJhellico]

Proposed change

Add Chile bank holidays (Jun 30 (1957-1975) and Dec 31 (since 1956)) and special "millennium" holiday Dec 31, 1999.

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/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 (d4eef1f) to head (173fdc1).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #1857   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files          184       184           
  Lines        11185     11194    +9     
  Branches      1763      1765    +2     
=========================================
+ Hits         11185     11194    +9     

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[arkid15r]

LGTM