LibWeb: Apply animation-timing-function per keyframe interval

Per the CSS Animations spec, the animation-timing-function property
describes how the animation progresses between each pair of keyframes,
not as an overall effect-level timing function.

Previously we set it as the effect-level timing function on the
AnimationEffect, which caused easing to be applied to the global
animation progress. This made animations with multiple keyframes
"pause" at the start and end of the full animation cycle instead of
easing smoothly between each pair of keyframes.

Now we:
- Store per-keyframe easing in ResolvedKeyFrame from @keyframes rules
- Store the default easing on CSSAnimation instead of on the effect
- Apply per-keyframe easing to the interval progress during
  interpolation, falling back to the CSS animation's default easing
- Also store per-keyframe easing from JS-created KeyframeEffects to
  avoid incorrectly applying CSS default easing to replaced effects

Tests/LibWeb/Text/expected/css/animation-timing-function-per-keyframe.txt

@@ -1,5 +1,5 @@
 linear at 0.25: 50px
 linear at 0.5: 100px
 ease at 0.25 is NOT linear: true
-ease at 0.5: 160.484375px
-per-keyframe linear at 0.25: 81.703125px
+ease at 0.5: 100px
+per-keyframe linear at 0.25: 50px