Only use RawReader on Windows if stdin is a console

Fixes #229 FYI: This uses the `syscall.SyscallN` function (instead of the deprecated Syscall5, etc.) to simplify the code a bit. That function was introduced in go1.18. I'm not sure if there is any desire to support compiling this library on older versions of go, but if there is I can switch it back to the deprecated methods.