Create an *end* anchor of a span

[uliska]

If the span applies to a sequential music expressions there should be a way to mark its end too.

Somehow the first anchor should hold a reference to that end anchor, from which the annotationEngraver should find a way to create an additional attribute for the end of the annotation.

However, I'm not 100% sure this is even possible because I don't know if in the engraver's finalize clause we have the necessary information about the other anchor's grob.
Maybe it can be made possible by reversing the responsibility by doing it from the end. Or by storing grobs along with the annotations in the final stage of the engraver.