v0.1.0 of thai_holidays

Pure-Dart Thai public-holiday calendar: fixed and movable Buddhist holidays, in-lieu days, special bridge days, and business-day helpers. Changelog excerpt: Initial release. ### Holidays - Fixed-date Thai public holidays (New Year, Chakri, Songkran ×3, Labour Day, Coronation Day, royal birthdays, Chulalongkorn Day, memorial days, Constitution Day, New Year's Eve), with historical effective-year gating: - King Vajiralongkorn's Birthday (28 Jul) — from 2017. - King Bhumibol Memorial Day (13 Oct) — from 2017. - Coronation Day (4 May) — from 2019. - Queen Suthida's Birthday (3 Jun) — from 2019. - Movable Buddhist holidays computed via `t[...]