Add translation, testcase for Berlin's 80th Liberation Day

[PPsyrius]

Proposed change

Hi, I've switched your coding implementation to the special_holiday category which is more suitable for one-time holidays like this, as well as adding translation, testcase, and snapshots for it.

Thanks again for your contribution :)

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