gh-131146: Fix month names in a "standalone form" in calendar module (GH-131147)

The calendar module displays month names in some locales using the genitive case.
This is grammatically incorrect, as the nominative case should be used when the month
is named by itself. To address this issue, this change introduces new lists
`standalone_month_name` and `standalone_month_abbr` that contain month names in
the nominative case -- or more generally, in the form that should be used to
name the month itself, rather than form a date.

The module now uses the `%OB` format specifier to get month names in this form
where available.