Added a Grey trait, and implementations on baked-in colors. Fixes #13206

[Earthmark]

Added a Grey trait to allow colors to create a generic "grey" color.

This currently assumes the color spaces follow the same gradient, which I'm pretty sure isn't true, but it should make a "grey-ish" color relative to the provided intensity.

Objective

• Implements #13206

Solution

• A small Grey trait was added and implemented for the common color kinds.

Testing

• Currently untested, unit tests exposed the non-linear relation between colors. I am debating adding an example to show this, as I have no idea what color space represents what relation of grey, and I figure others may be similarly confused.

Changelog

• The Grey trait was added, and the corresponding grey

BREAKING CHANGES

The const qualifier for LinearRGBA::gray was removed (the symbol still exists via a trait, it's just not const anymore)

[BD103]

Some colors have const fn gray() already. The old method should probably be removed if this trait is kept, though currently trait methods are non-const.

[bushrat011899]

Probably a good idea to include a little generic test.

[Earthmark]

I added a test to verify all the different colors implement white and black as gray(1) and gray(0) respectively.

I'm very tempted to add a trait for approximate equality on colors now, but for now this works. It's not approximate, so it may be error prone, but it seems to work for now...

[alice-i-cecile]

@Earthmark could you resolve the merge conflicts? I'll merge this in as soon as you do.

[Earthmark]

Will do. I'll get the conflicts resolved later today, and verify this is in prelude.

[Earthmark]

I forgot about this for a few days, but now it's here!

[alice-i-cecile]

Thanks!