LibWeb: Implement CSSNestedDeclarations type

This is basically a list of properties, without a block around it. It'll
be part of CSSStyleRules' contents.

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -2664,6 +2664,7 @@ void StyleComputer::build_qualified_layer_names_cache()
             case CSSRule::Type::Keyframes:
             case CSSRule::Type::Keyframe:
             case CSSRule::Type::Namespace:
+            case CSSRule::Type::NestedDeclarations:
             case CSSRule::Type::Supports:
                 break;
             }