Issue 4800: Require path separators in assignments/overrides

Issue 4800: Require path separators in assignments/overrides

Historical assignments/overrides like
Variable #'x #'y = 7
or
\override Glissando.bound-details #'left #'Y = #3
without dots between the various path components do not match the syntax of
\revert and music functions or other uses.  Disallow the non-separation
by '.' or ',' of such path components.