Support volume-break-{before,after}:avoid

[bertfrees]

Since there is already a kind of similar feature available in Dotify, namely keep-with-next-sheets, I don't consider it likely that Joel will add support for this.

Supporting it in css-to-obfl via existing features is not possible either. E.g. a volume-keep-priority on a wrapper element would affect the two whole elements and not just the point in between.