add @vx/voronoi

[williaster]

Voronoi diagrams are commonly used to improve the interaction experience of visualizations by enabling a wider mouse interaction area, and can be useful for the implementation of tooltips so this will hopefully be useful for #44.

This PR:

• adds the @vx/voronoi package
• adds a static voronoi tile example in /gallery
• adds an interactive voronoi page example linked from the tile at /voronoi that shows a proof-of-concept for how the package can be used for mouse interactions
• adds voronoi jest tests
• adds a voronoi readme with an overview of the package

The readme.md has an overview of what the package exports, basically a wrapper around d3-voronoi to expose the layout itself + a VoronoiPolygon that knows how to render the polygon.

/gallery tile

/voronoi page

Tested by:
tests run with npm run tests within packages/vx-voronoi/
/gallery and /voronoi tested locally in the packages/demo repo

[hshoff]

😍 🏆

This is going to be great for use with tooltips