Color conversion functions for shaders

[viridia]

What problem does this solve or what need does it fill?

Part of @ickshonpe 's recent work on shaders involved coding up WGSL color conversion functions for HSL, SRGBA, and other color spaces. Currently these functions are part of the gradient shader; however I would like to use these same functions in other shaders, particularly the "color plane" shader for the feathers color picker.

What solution would you like?

I'd like to move the color conversion functions somewhere that they can be used by multiple shaders. Ideally, I should just be able to import them into my shader code like the other Bevy pbr functions.

What alternative(s) have you considered?

Cutting and pasting the conversion functions into my own shader.

Additional context

#20010 #20035