The KeyframeList class is used almost exclusively in the animations code, so it would make sense to move it under the "animation" directory. We also refer to such keyframes as "blending keyframes" in the animation code, so we take this opportunity to also rename `KeyframeList` as `BlendingKeyframes` and `KeyframeValue` as `BlendingKeyframe`.
<rdar://problem/119663662>
Pull request: https://github.com/WebKit/WebKit/pull/21795
Committed 272055@main (11d1cdb0666c): <https://commits.webkit.org/272055@main> Reviewed commits have been landed. Closing PR #21795 and removing active labels.