LibWeb: Don't crash on Element.setAttribute() with emoji in name

We were mistakenly using StringBuilder's append(char) when we really
wanted append_code_point(u32).

Userland/Libraries/LibWeb/Infra/Strings.cpp

@@ -99,7 +99,7 @@ ErrorOr<String> to_ascii_lowercase(StringView string)
     auto utf8_view = Utf8View { string };
     for (u32 code_point : utf8_view) {
         code_point = AK::to_ascii_lowercase(code_point);
-        TRY(string_builder.try_append(code_point));
+        string_builder.append_code_point(code_point);
     }
     return string_builder.to_string();
 }