LibWeb: Add ConicGradientStyleValue

This commit adds a simple style value (which is an abstract image)
to represent conic-gradient()s.

This commit also starts to factor out some reusable parts of the
linear-gradient() style value for other gradient types.

Userland/Libraries/LibWeb/Forward.h

@@ -36,6 +36,7 @@ class BorderStyleValue;
 class Clip;
 class CalculatedStyleValue;
 class ColorStyleValue;
+class ConicGradientStyleValue;
 class ContentStyleValue;
 class CSSConditionRule;
 class CSSGroupingRule;