Use enums for month and weekday names

[lauft]

Using enums instead of strings for month and weekday names should improve type safety and IDE support.

[Copilot]

Pull request overview

This PR refactors the date handling utilities to use enums (Weekday and Month) instead of string literals for month and weekday names, improving type safety and IDE support across the entire codebase.

Key changes:

• Introduced Weekday(IntEnum) and Month(IntEnum) to replace string-based weekday/month representations
• Removed the SmartDayArrow.weekday() method that returned string weekday names (now uses Arrow.weekday() directly which returns integers)
• Updated all function signatures in utils to accept enum types instead of strings
• Migrated all holiday definition files across 30+ countries/regions to use the new enum types
• Added comprehensive test coverage for the enum functionality

Reviewed changes

Copilot reviewed 76 out of 76 changed files in this pull request and generated no comments.

File	Description
src/holidata/utils.py	Core refactoring: added Weekday/Month enums, removed string constants, updated type signatures, removed custom weekday() method
tests/test_utils.py	New comprehensive test file covering enum values and all utility functions with enum usage
src/holidata/holidays/*/*.py	Multiple country/region holiday definition files updated to use Month and Weekday enums instead of strings

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