MF2: ICU4C needs spec updates

MF2 in ICU4C needs to be updated to conform with the following spec changes:

• Match on variables rather than expressions --
  see https://github.com/unicode-org/message-format-wg/pull/877

• Require attribute values to be literals (if present) --
  see https://github.com/unicode-org/message-format-wg/pull/894

• Implement test:format, test:select, and test:function functions, to enable running the tests in https://github.com/unicode-org/message-format-wg/blob/main/test/tests/pattern-selection.json

  • see https://github.com/unicode-org/message-format-wg/blob/main/test/README.md#test-functions