Add which-key support

[fbrosda]

This adds support for the which-key-mode based on the advice in https://github.com/justbur/emacs-which-key/pull/353#issuecomment-1965404948

It reuses most parts of the original implementation found in https://github.com/justbur/emacs-which-key/pull/353/files and adjusts it for the changes made in https://github.com/justbur/emacs-which-key/commit/5fbdf05351e77ee62e3933c7b5f46459693bd04c

Related Issue: #3