Add ability for a private Calendar

[shaynesanderson-zz]

Feature Request

Add ability to mark a Calendar as "Private". When doing so, the calendar would have to be displayed using a shortcode with a new parameter maybe.

Any Calendar that was marked as "Private" would not show in the Calendar dropdown list that appears on all front-end Calendar displays.

[JJJ]

Achievable!

See: https://github.com/stuttter/wp-term-visibility

There also does not appear to be (that I have found) any per-calendar visibility setting in the iCalendar RFC, which is a good thing for this feature request, in that we have more freedom to make this work however we choose.

See: https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.1.3