Along/across-stroke gradients

There are two directions:

- Along the stroke

[See this](https://dev.to/uuuuuulala/gradient-along-svg-path-with-gsap-kl4) for info about how to approximate this. We can also just split up the stroke into a lot of pieces with solids colors, or with linear gradients aiming to go from the start to end, using greater subdivision where the start and end planes bend more quickly to aim for them to be close to parallel so the linear gradient works.


- Across the stroke

Repeat the stroke many times with increasing thickness. Use the technique described in #2268 for not mirroring at the centerline.


Regardless of method or direction, masking is likely necessary to handle end caps properly.