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/Forward.h

@@ -68,16 +68,12 @@ class BackgroundStyleValue;
 class BorderRadiusShorthandStyleValue;
 class BorderRadiusStyleValue;
 class BorderStyleValue;
-class CalculatedStyleValue;
-class Clip;
-class ColorStyleValue;
-class CompositeStyleValue;
-class ConicGradientStyleValue;
-class ContentStyleValue;
 class CSSConditionRule;
 class CSSFontFaceRule;
 class CSSGroupingRule;
 class CSSImportRule;
+class CSSKeyframeRule;
+class CSSKeyframesRule;
 class CSSMediaRule;
 class CSSRule;
 class CSSRuleList;
@@ -85,6 +81,12 @@ class CSSStyleDeclaration;
 class CSSStyleRule;
 class CSSStyleSheet;
 class CSSSupportsRule;
+class CalculatedStyleValue;
+class Clip;
+class ColorStyleValue;
+class CompositeStyleValue;
+class ConicGradientStyleValue;
+class ContentStyleValue;
 class CustomIdentStyleValue;
 class Display;
 class DisplayStyleValue;
@@ -157,10 +159,10 @@ class TimeOrCalculated;
 class TimePercentage;
 class TimeStyleValue;
 class TransformationStyleValue;
+class URLStyleValue;
 class UnicodeRange;
 class UnresolvedStyleValue;
 class UnsetStyleValue;
-class URLStyleValue;
 
 enum class MediaFeatureID;
 enum class PropertyID;