add \CompressMMRests command

[fedelibre]

LilyPond documentation actually prefers \compressMMRests over the currently available \compressFullBarRests. Details explained in LilyPond issue 3687.

[sincere-music]

I understand that Frescobaldi syntax completion is never updated to any but stable releases of LilyPond.

[uliska]

But there has been discussion (and in principle agreement) that there should be two sets of completion/highlighting info, for current stable and development versions.