ENH: Add scale for divergence/mutations

[corneliusroemer]

I realized that there doesn't seem to be a divergence/mutation scale in taxonium at the moment.

This would be incredibly useful, in the same way that maps show scales.

I'd probably display both, mutations and divergence. One could in principle add vertical gridlines, too, the way that Auspice does, in addition to a scale like |----5 muts---| --- 10 muts ---|