YYYY-MM[u-ca=iso8601] and MM-DD[u-ca=iso8601] formats are not round-trippable

[ptomato]

While answering #1970 I realized that in PlainYearMonth/PlainMonthDay's toString() methods, adding the calendarName: 'always' option does not print the reference day / reference year when the calendar is ISO:

> Temporal.Now.plainDateISO().toPlainMonthDay().toString({ calendarName: 'always' })
'12-10[u-ca=iso8601]'

I could not remember, despite reading our discussion in #703 (comment), whether this was intentional or not. It seems that at least at one point we didn't consider this out of the ordinary, because we added tests for the current behaviour in efb46b7.

However, this format is not round-trippable:

> Temporal.PlainMonthDay.from('12-10[u-ca=iso8601]')
// RangeError: invalid ISO 8601 string: 12-10[u-ca=iso8601]

(The same goes for PlainYearMonth.)

I'm not sure anything needs to change, since the output of toString() without any options is still round-trippable, and that's the thing that we've considered important. But I opened this issue to determine whether this MM-DD[calendar] format and its non-round-trippability was intentional or an oversight.

[sffc]

In my opinion, we should make { calendarName: 'always' } imply the full year-month-day syntax.

[justingrant]

Meeting 2022-01-20: toString of PlainYearMonth and PlainMonthDay should emit the full date (including the reference day or year, respectively) whenever we add a calendar annotation.