ISO week year (%g and %G) parsing and formatting is inconsistent.

[mbostock]

In #52, %g and %G were implemented as aliases of %y and %Y. This leads to inconsistencies with formatting. For example, 21-Jan parsed with %g-%b is 2021-01-01. This same date formatted with %g-%b is 20-Jan because the ISO week year 2021 doesn’t start until 2021-01-04. Therefore it would be more appropriate for the parser to have returned 2021-01-04. But that requires some fancier parsing logic…