ENH: Show aa / spike mutations on branches

[corneliusroemer]

One very important thing I'm missing from Taxonium is annotation of mutations on branches.

Hovering over branches for tooltips to pop up is tiring.

It would be nice if you could show them if there's sufficient space above them, say 50 pixels or something that's configurable.

Would be good to be able to select: show all, including nt, show only AA, show only a gene, show only a range of a gene (e.g. S:1-600)