LibWeb/CSS: Implement the color-scheme CSS property

Author: https://github.com/Gingeh
Commit: ce5cd012b9
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3146
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/Forward.h

@@ -116,6 +116,7 @@ class BasicShapeStyleValue;
 class BorderRadiusStyleValue;
 class CalculatedStyleValue;
 class Clip;
+class ColorSchemeStyleValue;
 class ConicGradientStyleValue;
 class ContentStyleValue;
 class CounterDefinitionsStyleValue;