C++20 feature: jthread

[nmcclatchey]

Issue tracking implementation of jthreads, introduced in C++20.

See jthread on cppreference.com.

[Jamaika1]

https://github.com/josuttis/jthread
I have no problem creating std::jthread, std::stop_token and mingw::thread in test_jthread1.cpp for gcc 11.3.0 in C++2a.
I use https://github.com/Jamaika1/mingw_std_threads