LibWeb/CSS: Merge RotationStyleValue into TransformationStyleValue

Same again, although rotation is more complicated: `rotate`
is "equivalent to" multiple different transform function depending on
its arguments. So we can parse as one of those instead of the full
`rotate3d()`, but then need to handle this when serializing.

Libraries/LibWeb/Forward.h

@@ -206,7 +206,6 @@ class RectStyleValue;
 class Resolution;
 class ResolutionOrCalculated;
 class ResolutionStyleValue;
-class RotationStyleValue;
 class Screen;
 class ScreenOrientation;
 class ScrollbarGutterStyleValue;