Proposal: a way to _not_ reverse an animation's timing function when it plays in `reverse` or `alternate` direction

[benface]

animation-direction: alternate is a nice way to write less keyframes when the second half of the animation is essentially a mirror of the first half, but it doesn't always mean we want the timing function to reverse in the second part. I wonder if we could add a flag to the upcoming animation-easing property to make that customizable?