LibWeb: Parse :before and :after pseudo-elements

Note that this is the old CSS2 syntax, we don't support the CSS3 syntax
just yet. Also we don't actually implement the pseudo-elements, this is
really just to make the selectors distinct from the same ones without
these pseudo-elements.

Libraries/LibWeb/CSS/Selector.h

@@ -57,6 +57,13 @@ public:
         };
         PseudoClass pseudo_class { PseudoClass::None };
 
+        enum class PseudoElement {
+            None,
+            Before,
+            After,
+        };
+        PseudoElement pseudo_element { PseudoElement::None };
+
         FlyString value;
 
         enum class AttributeMatchType {