Add support for conic gradients

[kkoreilly]

Describe the feature

These gradients shift by angle (instead of distance like radial gradients). See https://developer.mozilla.org/en-US/docs/Web/CSS/gradient/conic-gradient. This is low priority until someone needs it; I think that the color picker is actually pretty good without it, although we could reconsider that when we implement this; there are no urgent needs for this as of now.

Relevant code

No response