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.