LibWeb: Add preliminary support for CSS animations

This partially implements CSS-Animations-1 (though there are references
to CSS-Animations-2).
Current limitations:
- Multi-selector keyframes are not supported.
- Most animation properties are ignored.
- Timing functions are not applied.
- Non-absolute values are not interpolated unless the target is also of
  the same non-absolute type (e.g. 10% -> 25%, but not 10% -> 20px).
- The JavaScript interface is left as an exercise for the next poor soul
  looking at this code.

With those said, this commit implements:
- Interpolation for most common types
- Proper keyframe resolution (including the synthetic from-keyframe
  containing the initial state)
- Properly driven animations, and proper style invalidation

Co-Authored-By: Andreas Kling <kling@serenityos.org>

Userland/Libraries/LibWeb/CSS/CSSConditionRule.cpp

@@ -22,6 +22,12 @@ void CSSConditionRule::for_each_effective_style_rule(Function<void(CSSStyleRule
         CSSGroupingRule::for_each_effective_style_rule(callback);
 }
 
+void CSSConditionRule::for_each_effective_keyframes_at_rule(Function<void(CSSKeyframesRule const&)> const& callback) const
+{
+    if (condition_matches())
+        CSSGroupingRule::for_each_effective_keyframes_at_rule(callback);
+}
+
 JS::ThrowCompletionOr<void> CSSConditionRule::initialize(JS::Realm& realm)
 {
     MUST_OR_THROW_OOM(Base::initialize(realm));