CreateFile2 in mmap::open fails with UTF-8 string in Unicode env

[Colysia]

#1973
In this PR, the MultiByteToWideChar function is used to convert a std::string to a std::wstring. This works fine when the C++ project is using the default multibyte character set in Windows. However, in cases where we choose to use UTF-8 encoding for the source files and configure the C++ project to use the Unicode character set, this approach might not work as expected. Therefore, I would suggest providing an option to set a callback, allowing developers to implement their own conversion logic from std::string to std::wstring based on their specific encoding environment.