Periodically check if exercises are in sync via GH Action

[fapdash]

Found this action on the common-lisp track and it seems very useful to keep a track healthy and up-to-date.

See https://github.com/exercism/common-lisp/blob/de5fadcd239987563f43b67c23863afe485a954d/.github/workflows/configlet-sync.yml

I tested the action on my fork, see

• https://github.com/fapdash/exercism--emacs-lisp/actions/runs/3503328179/jobs/5868317315 (failure is expected since the exercises are currently not in sync)
• https://github.com/fapdash/exercism--emacs-lisp/issues/3