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/Parser/DeprecatedCSSParser.cpp

@@ -579,6 +579,9 @@ public:
                 simple_selector.pseudo_class = CSS::Selector::SimpleSelector::PseudoClass::Enabled;
             } else if (pseudo_name.equals_ignoring_case("checked")) {
                 simple_selector.pseudo_class = CSS::Selector::SimpleSelector::PseudoClass::Checked;
+            } else if (pseudo_name.starts_with("not", CaseSensitivity::CaseInsensitive)) {
+                simple_selector.pseudo_class = CSS::Selector::SimpleSelector::PseudoClass::Not;
+                simple_selector.not_selector = capture_selector_args(pseudo_name);
             } else {
                 dbgln("Unknown pseudo class: '{}'", pseudo_name);
                 return {};

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 {

Userland/Libraries/LibWeb/Dump.cpp

@@ -384,6 +384,9 @@ void dump_selector(StringBuilder& builder, const CSS::Selector& selector)
             case CSS::Selector::SimpleSelector::PseudoClass::Checked:
                 pseudo_class_description = "Checked";
                 break;
+            case CSS::Selector::SimpleSelector::PseudoClass::Not:
+                pseudo_class_description = "Not";
+                break;
             }
 
             builder.appendff("{}:{}", type_description, simple_selector.value);