Editorial: Provide override of NonISODateLastDayOfMonth

[ptomato]

See tc39/proposal-temporal#3208. This would be the corresponding definition of NonISODateLastDayOfMonth using the calendar-specific operations defined in this proposal.

[sffc]

Do you plan to use the new AO in the multiple places in spec.emu that call BalanceNonISODate to achieve this goal?

[ptomato]

I looked at doing that. This operation needs to convert from ISO date to calendar date and back, whereas the other two places where we call BalanceNonISODate don't need that. Of course, implementations can just elide those conversions, or we could refactor things so that it made more sense to deduplicate the BalanceNonISODate calls. But I didn't want to put too much time in it right at this moment since I'm not 100% sure this PR is necessary for tc39/proposal-temporal#3208

[ptomato]

Assuming tc39/proposal-temporal#3253 is accepted, we won't need this. If we still want to make a separate last-day-of-month operation, I can add that later.