feat: Linear gradient support in CSS animations

[MatiPl01]

Summary

Test plan

[MatiPl01]

Superseded by #10193, which adds animated backgroundImage gradients (linear and radial) across useAnimatedStyle, CSS animations and CSS transitions, with the C++ interpolators to match.

This draft only covered linear-gradient and predates the css/platform/native -> css/native move, so there is nothing left worth rebasing. Closing.