Thailand - add new country support

[xdebbie]

Summary

• Add holiday provider for Thailand (TH) with 16 fixed-date public holidays
• Register Thailand in HolidaySystem.cs

Holidays Included

National Holidays:

• New Year's Day (Jan 1)
• Chakri Memorial Day (Apr 6)
• Songkran Festival (Apr 13-15, 3 days)
• National Labour Day (May 1)
• Coronation Day (May 4, since 2019)
• H.M. Queen Suthida's Birthday (Jun 3, since 2019)
• H.M. King Vajiralongkorn's Birthday (Jul 28)
• H.M. Queen Mother's Birthday / Mother's Day (Aug 12)
• Prince Mahidol Day (Sep 24)
• H.M. King Bhumibol Memorial Day (Oct 13)
• Chulalongkorn Day (Oct 23)
• H.M. King Bhumibol's Birthday / Father's Day (Dec 5)
• Constitution Day (Dec 10)
• New Year's Eve (Dec 31)

Not included (lunar calendar-based):

• Makha Bucha Day
• Visakha Bucha Day
• Asahna Bucha Day
• Royal Ploughing Ceremony

Implementation Notes

• Coronation Day and Queen Suthida's Birthday are conditionally added for years >= 2019
• All holiday names include Thai local names (e.g., วันสงกรานต์)

Test plan

• Verify holidays render correctly for years before and after 2019
• Verify Songkran displays as 3 separate days (Apr 13, 14, 15)

[tinohager]

I looked at the holidays and did some research. Unfortunately, the information does not seem to be entirely correct. Therefore, a detailed review of the information is required.