Refactor grammar to give \addlyrics a good target to attach to

Refactor grammar to give \addlyrics a good target to attach to

So far,

    \new Staff \new Voice { c1 } \addlyrics { Oh } \addlyrics { Ah }

was factored as

    \new Staff \new Voice { { c1 } \addlyrics { Oh } \addlyrics { Ah } }

which resulted in something like

   \new Staff \new Voice
     << \context Voice = "uniqueContext0" { c1 }
        \new Lyrics \lyricsto "uniqueContext0" { Oh }
        \new Lyrics \lyricsto "uniqueContext0" { Ah }
     >>

This made it impossible to work with the likes of

  \new Staff \new Voice \with { \stemUp } { c1 } \addlyrics ...

since the voice the lyrics attached to was always a new voice.
Fixing the grouping in the grammar is a first required step for getting
this under control.