[css-transitions][css-animations][web-animations] Add control of colorspace used for transitioning colors

( Spun out from #7035 )(similar to #7062)

Currently, all color transitions happen in gamma-encoded sRGB space. There is a need to preserve this as a default, for Web compat. However, CSS Color 4 defines a bunch of other color models, which exceed the very limited gamut of sRGB, and there is a need to add control of the color space used for interpolation which is currently unspecified:

• to avoid needless conversion to sRGB, dropping the more saturated colors
• because interpolating in gamma-encoded space gives overly dark midpoint colors
• because some color spaces give better results than others, which kind of varies depending what the start and end colors are, so stylesheet authors need the flexibility to choose.

As this is a common requirement, CSS Color 4 defines a <color-interpolation-method> token and associated color interpolation math; this has already been used in the grammar for `color-mix() in CSS Color 5 and in linear, radial and conic gradients in CSS Images 4.

So this issue proposes to add a new property, initial value auto:

transition-interpolation: [ auto | <color-interpolation-method> ]#

and also to add this to the <single-transition> shorthand. There is no ambiguity (unlike time) so it can go in any order in the shorthand.

Start by reading the CSS Color 4 interpolation-space definition and compare its use in CSS Color 5 color-mix() and CSS Images 4 gradients. Then inspect the CSS Transitions definitions for transition properties and the single-transition shorthand. Done means resolving the proposed color-space control and specifying its default and shorthand grammar.