Fix Windows encoding issue

[bschatt]

We uses CLI11 internally to build some of our tools. We are building with MSVC from VS2022, have enabled UNICODE on Windows, as suggested in the UTF-8 Everywhere manifesto and codecvt is available (CLI11_HAS_CODECVT is 1). With this setup we have been unable to parse non-ANSI parameters properly.

We have tracked the issue back to the narrow implementation. Specifically on Windows, codecvt_utf8_utf16 is used to convert str. Even though the type is wchar_t it is NOT an UTF-16 string. It's actually an UCS-2 string.

CLI11/include/CLI/impl/Encoding_inl.hpp

Line 65 in 3afddf3

return std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>>().to_bytes(str, str + str_size);

According to the documentation, codecvt_utf8<wchar_t> convert UTF-8 to/from UCS-2 on Windows and to/from UTF-32 on other platforms.

This probably fixes #1042