[css-color-4] Consider adding support for interpolation in the srgb-linear color space

[weinig]

Now that CSS Color Module 4 has support for linear sRGB, https://drafts.csswg.org/css-color/#predefined-sRGB-linear, I think it would be useful to extend the set of color spaces that can be used for interpolation to include it, https://drafts.csswg.org/css-color/#typedef-rectangular-color-space.

The main use case I can think of is consistent behavior with other parts of the stack (such as WebGL/WebGPU) that might be using linear sRGB.

[svgeesus]

This makes sense. The results should be identical to interpolating in xyz-d65 but indeed, srgb-linear was primarily added for consistency with other parts of the open web platform so it makes sense to allow it here too.