Provide `GetTempFileNameW`

[hasufell]

https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-gettempfilenamew

While working on a AFPP PR for the directory package I stumbled over the missing openBinaryTempFile function that's only defined for FilePath. Using Win32 bindings is probably easier than copy-pasting code from base.