Editorial: Add explicit step to reject too large dates in ToTemporalYearMonth

The call to `CalendarYearMonthFromFields` will already reject too large
dates, but adding an explicit step makes it more obvious that
`ISODateToFields` doesn't require support for mapping large ISO dates to
calendar dates. And it also aligns `ToTemporalYearMonth` with
`ToTemporalMonthDay` after #3054.

Author: anba

spec/plainyearmonth.html

@@ -480,6 +480,7 @@ <h1>
         1. Let _resolvedOptions_ be ? GetOptionsObject(_options_).
         1. Perform ? GetTemporalOverflowOption(_resolvedOptions_).
         1. Let _isoDate_ be CreateISODateRecord(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]).
+        1. If ISOYearMonthWithinLimits(_isoDate_) is *false*, throw a *RangeError* exception.
         1. Set _result_ to ISODateToFields(_calendar_, _isoDate_, ~year-month~).
         1. NOTE: The following operation is called with ~constrain~ regardless of the value of _overflow_, in order for the calendar to store a canonical value in the [[Day]] field of the [[ISODate]] internal slot of the result.
         1. Set _isoDate_ to ? CalendarYearMonthFromFields(_calendar_, _result_, ~constrain~).