Define Scheme markups using define-public (issue 577720043 by address@hidden)

[dak]

Retaining define-markup-command-internal in order to allow defining
markups in LilyPond syntax in a manner that stops being supported from
Scheme seems like incoherent design.

markup-lambdas can be created from within LilyPond using \markup ...
\etc .  Having them unusable from Scheme for the purpose of defining a
markup function seems inappropriate.

If unsupporting the functionality via define-markup-command is desirable
(which I am not convinced of) or independently, it would seem that a
more regular/prominent module-define-markup-command! with appropriate
semantics would be warranted (and used from the parser) instead of
retaining define-markup-command-internal as a non-internal of
define-markup-command.

https://codereview.appspot.com/577720043/