Crontab format parsing

Format follows that of the Vixie cron:
https://man7.org/linux/man-pages/man5/crontab.5.html

including support for the following Date-Time matching:
(month AND hour AND minute AND (day-of-month OR day-of-week))

Short 3-letter names for months and week days are supported,
e.g. Jan, Wed.

Sunday's day of week number can be either 0 or 7.

Date-Time entities use java.time.ZonedDateTime for UTC timezone.