ICU-11533 limited Myanmar calendar support

[mapmeld]

Background
A while ago I proposed PR #630 to add the Myanmar lunar-based calendar (aka Burmese calendar) to ICU. This calendar is frequently used for birthdays and on official documents.
After difficulties with supporting and testing the complete calendar and older dates, I closed the PR last year.

Goals
This new PR adds a limited Myanmar calendar that changes only the year; for example today (April 23, 2025 AD) would be represented as April 23, 1387 ME. This is similar to the modern Thai calendar (Buddhist / buddhcal in ICU).
A place in ICU and CLDR sets a foundation to support the months and days of the civil lunar cycle either in future updates or a region-specific API.

Representing Myanmar new year
The Myanmar new year (Thingyan) consistently falls in mid-April. This is possible by occurring at different dates or even different months on the complete Myanmar calendar; for example the current Myanmar year started on the 20th day of Tagu, and the next will start on the 2nd day of Kason.

This PR would increment years on April 17th:

• The year returned by this code is correct between April 17, 2000 AD and April 16, 2035 AD inclusive
• Other new year dates become more frequent over time, unless a secondary leap year (big watat) is declared
• By nature, a discrepancy caused by a different new year lasts for only one day
• Corrections for individual years are not feasible, for example April 16, 1362 ME could be reversed to either 2000 or 2001 AD.

[FrankYFTang]

I think it is a very bad idea to support an incomplete (and therefore nobody use) calendar into ICU. See https://www.frontiermyanmar.net/en/the-myanmar-calendar/ the Myanmar calendar is not Gregorian based.