bug#55140: 27.1; Feature request: allow customize holidays list

[José Júnior]

Hi, the feature request is an option in customize to allow adding/hiding holiday-lists.

Currently, the lists are hard coded here: https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/calendar/holidays.el#n437 and seems that the only way to customize would be overriding/wrapping the function.

This change would make it easier to create custom user-defined independent holiday lists, like seeing holidays of multiple regions, or religions, or anything, not only the ones that comes with emacs.