LibWeb: Implement @supports rule :^)

The main thing missing is that we don't serialize the supports clause,
but for actually using a `@supports (something: cool) {}` rule in CSS,
it works!

Userland/Libraries/LibWeb/Forward.h

@@ -26,6 +26,7 @@ class CSSRuleList;
 class CSSStyleDeclaration;
 class CSSStyleRule;
 class CSSStyleSheet;
+class CSSSupportsRule;
 class Display;
 class ElementInlineCSSStyleDeclaration;
 class Length;