time-stamp-time-zone: update customization

time-stamp-time-zone: update customization

* time-stamp.el (time-stamp-time-zone): Support customization with
an integer offset (a new possible value of the ZONE argument to
format-time-string in Emacs 27).
Update the safe-local-variable predicate from string-or-null-p
(describing time-stamp-time-zone's domain before 2015) to new
predicate time-stamp-zone-type-p (describing the current domain).

* time-stamp-tests.el (time-stamp-test-helper-zone-type-p): New test.