Feature: Voronoi Labels

[susielu]

Hat tip to @veltman for showing this to me.

It would be great it Semiotic had an annotation type that used this method outlined by @harrysevens:

Here's the full notebook of examples: https://observablehq.com/@harrystevens/directly-labelling-lines

Notes from our discussion:

• Could allow for this type to trigger a voronoi calculation without rendering if necessary
• Decorate the lines with this metadata when the voronoi is calculated
• Create a new type like how hightlight-annotation works