Feature Request: Extend widen and narrow to accept std::string view and similar view types

Feature Request: Extend widen and narrow to accept std::string view and similar view types: a task in LegoFlow-SWE (Harbor dataset). Currently, boost::nowide::widen() and boost::nowide::narrow() accept std::string , std::wstring , and raw pointers, but they do not accept std::string view …

The task

Currently, `boost::nowide::widen()` and `boost::nowide::narrow()` accept `std::string`, `std::wstring`, and raw pointers, but they do not accept `std::string_view`, `std::wstring_view`, `std::u16string_view`, `std::u32string_view`, or other string-like view types. This makes it impossible to use these conversion…

Part of Lego-X/LegoFlow-SWE.