[css-values-4][css-color-5] Define that color interpolation uses color-mix() if necessary

[tabatkins]

Currently, Values 4 defines simply that colors are interpolated in premultiplied sRGB. Colors 5 expands on this with more details (and we should edit Values 4 to defer to it, probably). However, neither of them actually say what happens when you interpolate between currentcolor and something else.

This can't be resolves at computed-value time, so I think the best answer is that it resolves to a color-mix().