time-stamp: add support for time zone numeric offset

* time-stamp.el: Implement %:z as expanding to the numeric time zone
offset, to address the feature request of bug#32931.  Do not document it
yet, to discourage compatibility problems in mixed Emacs 26 and Emacs 27
environments.  Documentation will be added in a subsequent release at
least two years later.  (We cannot yet use %z for numeric time zone
because in Emacs 26 it was documented to do something else.)

* time-stamp-tests.el (time-stamp-test-format-time-zone): expand this
test and break it into two tests, time-stamp-test-format-time-zone-name
and time-stamp-test-format-time-zone-offset.
1 job for master in 57 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #4129


Name Stage Failure
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1