P2845R8: Formatting of `std::filesystem::path`

[ldionne]

Link: https://wg21.link/P2845R8

When implementing this paper be aware of [format.formatter.spec]/3
"
Unless specified otherwise, for each type T for which a formatter specialization is provided by the library, each of the headers provides the following specialization: template<> inline constexpr bool enable_nonlocking_formatter_optimization = true;
"