LibWeb: Rough implementation of CSS namespace rule

This provides a rough implementation of the CSS @namespace rule.
Currently we just support default namespaces, namespace prefixes
are still to come.

Userland/Libraries/LibWeb/CSS/CSSRule.h

@@ -29,6 +29,7 @@ public:
         FontFace = 5,
         Keyframes = 7,
         Keyframe = 8,
+        Namespace = 10,
         Supports = 12,
     };