Add shader color conversion methods

[alice-i-cecile]

That makes sense. I've done something similar: my gradient slider UiMaterial shader interpolates in Srgba, because it looks much better that way.
I can envision a future version of Gizmos which does the interpolation in oklab (which, by many accounts, produces the best looking gradients).
Maybe we need to consider a library of color conversion functions written for shader code?

Originally posted by @viridia in #12162 (comment)