Issue 4537/1: Let \addlyrics accept an optional context mod

Issue 4537/1: Let \addlyrics accept an optional context mod

Since \addlyrics always creates a new Lyrics context, there seems to be
little point in not allowing it to have a \with clause.

This change allows the use of the syntax

    \new Voice {...} \addlyrics \with ... {...}

in order to achieve the effect of what previously required

   << \new Voice = "name" {...}
      \new Lyrics \with ... \lyricsto "name" {...}
   >>