LibWeb: Add 'PseudoElement' as a CSS SimpleSelector::Type

Same reasoning again! This is the last one.

While I was at it, I added the two remaining CSS2.2 pseudo-elements,
::first-line and ::first-letter. All 4 are handled in the new CSS
parser, including with the compatibility single-colon syntax. I have
not added support to the old parser.

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -24,6 +24,7 @@ public:
             Class,
             Attribute,
             PseudoClass,
+            PseudoElement,
         };
         Type type { Type::Invalid };
 
@@ -71,6 +72,8 @@ public:
             None,
             Before,
             After,
+            FirstLine,
+            FirstLetter,
         };
         PseudoElement pseudo_element { PseudoElement::None };