Convert between std::u8string and std::string

C++20 added char8_t and std::u8string for UTF-8. However, there is no UTF-8 version of std::cout and OS APIs mostly expect char and execution character set. So we still need a way to convert between UTF-8 and execution character set.

I was rereading a char8_t paper and it looks like the only way to convert between UTF-8 and ECS is to use std::c8rtomb and std::mbrtoc8 functions. However, their API is extremely confusing. Can someone provide an example code?