LibWeb: Implement :nth-child pseudo-class

Author: https://github.com/miere43
Commit: aa83539d7b
Pull-request: https://github.com/SerenityOS/serenity/pull/6968
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/Dump.cpp

@@ -351,6 +351,9 @@ void dump_selector(StringBuilder& builder, const CSS::Selector& selector)
             case CSS::Selector::SimpleSelector::PseudoClass::LastOfType:
                 pseudo_class_description = "LastOfType";
                 break;
+            case CSS::Selector::SimpleSelector::PseudoClass::NthChild:
+                pseudo_class_description = "NthChild";
+                break;
             case CSS::Selector::SimpleSelector::PseudoClass::Focus:
                 pseudo_class_description = "Focus";
                 break;