Add iCalendar export support

[kawazap]

Proposed change

This PR introduces a new iCalendar export functionality to the holidays library. The implementation includes:

• New ICalExporter class for converting holiday data to iCalendar format
• Support for basic iCalendar properties (VCALENDAR, VEVENT)
• Preparation for multi-language support through language parameter
• Test coverage for basic calendar structure and event generation

The exporter follows RFC 5545 specifications for iCalendar format, allowing users to easily export their holiday data to calendar applications.

Example usage:

from holidays import US
from holidays.ical import ICalExporter

us_holidays = US(years=2024)
exporter = ICalExporter(us_holidays)
ical_content = "\n".join(exporter.generate())

Resolves #2179

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 successfully run make check, all checks and tests are green

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4526e2a) to head (a24717a).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##               dev     #2261    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          196       200     +4     
  Lines        11935     12150   +215     
  Branches      1722      1732    +10     
==========================================
+ Hits         11935     12150   +215     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[KJhellico]

Preparation for multi-language support through language parameter

I think it would be better to use the built-in Holidays l10n instead of creating something specific for iCal.

[kawazap]

@KJhellico
Thank you for your review. you're absolutely right about using the built-in Holidays l10n system.
I am on TODO regarding it and will be implementing it later!