[m-rev.] for review: add is_leap_year/1 and days_in_month/2

[Zoltan Somogyi]

On Fri, 20 Mar 2026 19:32:45 +1100, Julien Fischer <jfischer at opturion.com> wrote:
> library/calendar.m:

Almost all of that is good, so commit it. I don't particularly like
either the documentation or the implementation of the leap year test,
but it is easier for me to make changes and post them for review
than to describe them.

Zoltan.