Add Spline segments / Trail

- Segments (variable color/style)
  - https://airbnb.io/visx/splitlinepath ([impl](https://github.com/airbnb/visx/blob/master/packages/visx-shape/src/shapes/SplitLinePath.tsx))
- Trail (variable width)
  - https://vega.github.io/vega/docs/marks/trail/
  - https://observablehq.com/@fil/trail-marks
  - https://svelteplot.dev/marks/trail
  - https://github.com/d3/d3-shape/pull/168