Add shader color conversion methods

>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 https://github.com/bevyengine/bevy/issues/12162#issuecomment-1970461738_