LibWeb: Port CSS/Serialize.{h,cpp} to new Strings, and propagate errors

Author: https://github.com/AtkinsSJ
Commit: 85f3de6331
Pull-request: https://github.com/SerenityOS/serenity/pull/17478
Reviewed-by: https://github.com/trflynn89 ✅

Userland/Libraries/LibWeb/CSS/Parser/Declaration.cpp

@@ -23,7 +23,7 @@ ErrorOr<String> Declaration::to_string() const
 {
     StringBuilder builder;
 
-    serialize_an_identifier(builder, m_name);
+    TRY(serialize_an_identifier(builder, m_name));
     TRY(builder.try_append(": "sv));
     TRY(builder.try_join(' ', m_values));