LibWeb/CSS: Resolve var() in keyframe animation-timing-function

When a `@keyframes` rule contains `animation-timing-function` with a
`var()`, we cannot eagerly resolve it to an `EasingFunction` at rule
cache build time because there is no element context available. We now
store the unresolved `StyleValue` and defer resolution to
`collect_animation_into()`, where the animated element's custom
properties can be used to substitute the variable. Previously, an
`animation-timing-function` with a `var()` in a `@keyframe` would cause
a crash.

Libraries/LibWeb/Animations/KeyframeEffect.h

@@ -70,7 +70,7 @@ public:
             // before they are applied to an element
             HashMap<CSS::PropertyID, Variant<UseInitial, NonnullRefPtr<CSS::StyleValue const>>> properties {};
             Bindings::CompositeOperationOrAuto composite { Bindings::CompositeOperationOrAuto::Auto };
-            Optional<CSS::EasingFunction> easing {};
+            Variant<Empty, CSS::EasingFunction, NonnullRefPtr<CSS::StyleValue const>> easing {};
         };
         RedBlackTree<u64, ResolvedKeyFrame> keyframes_by_key;
     };