LibWeb: Add :not pseudoclass to the CSS parser

The selector given in the :not() is stored in the SimpleSelector as a
String.

Userland/Libraries/LibWeb/CSS/Selector.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/FlyString.h>
+#include <AK/String.h>
 #include <AK/Vector.h>
 
 namespace Web::CSS {
@@ -41,6 +42,7 @@ public:
             Disabled,
             Enabled,
             Checked,
+            Not,
         };
         PseudoClass pseudo_class { PseudoClass::None };
 
@@ -75,6 +77,7 @@ public:
         // FIXME: We don't need this field on every single SimpleSelector, but it's also annoying to malloc it somewhere.
         // Only used when "pseudo_class" is "NthChild" or "NthLastChild".
         NthChildPattern nth_child_pattern;
+        String not_selector {};
     };
 
     struct ComplexSelector {