Move formatter<std::error_code> from fmt/os.h to fmt/std.h

[phprus]

Issue: #3123

[vitaut]

Thank you!

[tupaschoal]

Should this PR also have touched documentation for std.h? It currently does not say that it provides formatting for error_code, just:

fmt/std.h provides formatters for:

[std::filesystem::path](https://en.cppreference.com/w/cpp/filesystem/path)

[std::thread::id](https://en.cppreference.com/w/cpp/thread/thread/id)

[std::monostate](https://en.cppreference.com/w/cpp/utility/variant/monostate)

[std::variant](https://en.cppreference.com/w/cpp/utility/variant/variant)

[std::optional](https://en.cppreference.com/w/cpp/utility/optional)