ci: Add commit rule to not end subject with punctuation

I recently forgot to fill in the subject after the prefix. Add a
commit rule for catching this and other cases where subject lines
end in punctuation (except ...)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1722>

.gitlab-ci/commit-rules.yml

@@ -11,3 +11,6 @@ patterns:
     - regex: '^\S*\.(js|c|h):'
       message: Commit message subject prefix should not include .c, .h etc.
       where: subject
+    - regex: '([^.]\.|[:,;])\s*$'
+      message: Commit message subject should not end with punctuation
+      where: subject