Add Okhwba color space

[tmstorey]

Objective

Bevy has multiple Ok color spaces and the HWB color space, but not the Okhwb color space. This would be good for consistency, and also useful for supporting triangular color pickers, such as an Ok color space version of the HWB picker in my other PR: #24917.

Solution

• Adds Okhwba color space struct, with new, hwb, with_whiteness and with_blackness methods.
• Derives Debug, Clone, Copy, PartialEq, Reflect, and serialization.
• Implements StandardColor, Default, Mix, Gray, Alpha, Hue, ColorToComponents and various From impls.
• Adds Color::Okhwba(Okhwba) enum variant, and Color::okhwba() and Color::okhwb() constructors.
• Updates conversion diagram MMD and SVG.

Testing

• Tests mostly copied from Okhsva.
• Tests using gen_tests and TEST_COLORS not yet implemented.
• All current tests pass.

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Showcase

New conversion diagram:

[kfc35]

are you planning to do the gen_tests changes in this PR as well? the description mentions it but it’s not clear whether this PR is waiting for that

Beyond that, this PR looks mostly good except for the one thing re: testing okhsva <-> okhwba conversion I mentioned

[kfc35]

Instead of this being copied from okhsva as is, I would think there needs to be a thorough test like this one that ensures the conversion between okhwba and okhsva is as expected, since that’s the new logic implemented in this module