Issue 5167/5: Allow defining markup commands in LilyPond syntax

Issue 5167/5: Allow defining markup commands in LilyPond syntax

This works with assignments of the form

    \markup with-red = \markup \with-color #red \etc

or similar.  The resulting definition (in addition to being available
as \with-red command inside of markups) can be used with the `markup'
macro and also gets a `make-with-red-markup' convenience function.