string: add is_transparent to transforming less comparator

string.hpp

@@ -350,6 +350,8 @@ namespace cruft::string {
             return false;
         }
 
+        using is_transparent = void;
+
         template <typename A, typename B>
         requires (!std::is_same_v<A,B>)
         bool operator() (A &&a, B &&b) const noexcept