Bring BalanceYearMonth in line with spec
Bring BalanceYearMonth in line with spec

The previous code would produce wrong values when subtracting a duration
of more than 12 months that wasn't specified in years, e.g. P14M2D. The
algorithm in the spec seems to be correct.

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill