Add: AppendStringInPlace() to append translated string ID into an existing string. (#12969)

This allows avoiding a string copy when building strings.

src/strings_func.h

@@ -61,6 +61,7 @@ inline StringID MakeStringID(StringTab tab, uint index)
 
 std::string GetString(StringID string);
 const char *GetStringPtr(StringID string);
+void AppendStringInPlace(std::string &result, StringID string);
 
 uint ConvertKmhishSpeedToDisplaySpeed(uint speed, VehicleType type);
 uint ConvertDisplaySpeedToKmhishSpeed(uint speed, VehicleType type);