move QString-formatting to helpers.hpp so fS works with it all the time

src/util/helpers.hpp

@@ -11,3 +11,13 @@ auto fS(Args &&... args) -> decltype(fmt::format(std::forward<Args>(args)...))
 }
 
 }  // namespace chatterino
+
+namespace fmt {
+
+// format_arg for QString
+inline void format_arg(BasicFormatter<char> &f, const char *&, const QString &v)
+{
+    f.writer().write("\"{}\"", v.toStdString());
+}
+
+}  // namespace fmt