[FEEDBACK] Resolution between input calendar and message calendar

[sffc]

This is similar to https://github.com/unicode-org/message-format-wg/issues/961, but about calendars.

There could be two sources for calendars: the input type being formatted, and the message string. (Actually, there is a third, the locale of the message formatter.)

The spec should be more clear about resolution when these are in conflict with one another.

As a reference point, in Temporal, we throw an exception if two of the calendars differ and they are not ISO-8601. For example:

Calendar 1	Calendar 2	Resolved
iso8601	iso8601	iso8601
iso8601	buddhist	buddhist
buddhist	chinese	ERROR