LibWeb: Parse and match the :visited pseudo-class (always fails)

If we don't do this, something like "a:visited" is parsed as "a" which
may then take precedence over a previous "a:link" etc.

Libraries/LibWeb/CSS/Selector.h

@@ -47,6 +47,7 @@ public:
         enum class PseudoClass {
             None,
             Link,
+            Visited,
             Hover,
             Focus,
             FirstChild,