MISC: Fix the StringType concept (operator[] should return a const&)

src/Lib/Utils.hh

@@ -73,7 +73,7 @@ concept StringType = requires(T str)
     { str.size() }  -> std::convertible_to<std::size_t>;
     { str.begin() } -> std::same_as<typename T::iterator>;
     { str.end() }   -> std::same_as<typename T::iterator>;
-    { str[0] }      -> std::same_as<typename T::value_type>;
+    { str[0] }      -> std::same_as<typename T::const_reference>;
     // clang-format on
 };
 }