LibHTML: Parse the :link and :hover CSS pseudo-classes

We don't actually do anything with these yet, but now the values will
be there for the selector engine to look at when it feels ready. :^)

Libraries/LibHTML/CSS/Selector.h

@@ -15,6 +15,13 @@ public:
         };
         Type type { Type::Invalid };
 
+        enum class PseudoClass {
+            None,
+            Link,
+            Hover,
+        };
+        PseudoClass pseudo_class { PseudoClass::None };
+
         enum class Relation {
             None,
             ImmediateChild,