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/CSS/CSSRule.h

@@ -26,6 +26,7 @@ public:
         Style,
         Import,
         Media,
+        Supports,
         __Count,
     };