bug#46736: Add gnus-article-date-emacs-calendar

[積丹尼 Dan Jacobson]

Noting
W T e           gnus-article-date-lapsed
W T i           gnus-article-date-iso8601
W T l           gnus-article-date-local
W T o           gnus-article-date-original
W T p           gnus-article-date-english
W T s           gnus-article-date-user
W T u           gnus-article-date-ut
W T z           gnus-article-date-ut
and
(info "(emacs) Calendar Systems")
one thinks, ah ha, why aren't these tied together, with a
W T x ...       gnus-article-date-emacs-calendar
which would fetch the date via emacs' calendar system,
(Where each letter was a different calendar system.)
So
W T x a
would show your mail's date in the Azerbaijani calendar,
W T x b
would show your mail's date in the Bulgarian calendar, etc.

Some real examples:

Here in Taiwan, all official documents use 110, not 2021, so such would
be very useful for people who want to think in one system when reading a
mail.

Same in Japan. It's not "only 2021" officially.