beam rule: don't include rests in a beam

created a new type of beam rule function:


    // BeamRule determines what we do when we evaluate two adjacent notes
    enum BeamRule {
        case split // separate them
        case drop // drop both
        case keep // keep together
    }

    private static let beamRulesV2: [(NoteViewModel, NoteViewModel) -> BeamRule] = [
        { $0.note.rest || $1.note.rest ? .split : .keep },
    ]

btc / allegro

beam rule: don't include rests in a beam #283