Not tested, but from reading: very nice. LGTM
update call-site in parser.yy
commit 9f1683921621b612b94080d506ee317b058b29c8 Author: Han-Wen Nienhuys <hanwen@lilypond.org> Date: Sat Feb 15 12:36:32 2020 +0100 Express define-markup-list-command-internal using define-markup-command-internal