feat: allow quoted TZ and CRON_TZ values in spec strings (#335)
Shell users habitually write TZ="America/New_York" or TZ='UTC' when
specifying timezones. Previously these were rejected because quote
characters are not valid in IANA timezone names.
Now matching single or double quotes are stripped from the timezone
value before validation. Mismatched or partial quotes still produce
clear errors.
Signed-off-by: Sebastian Mendel <info@sebastianmendel.de>