fix(utils): improve path separator detection on Windows

[ayooh]

If you use neovim binary from MSYS2 packages, the vim.loop.os_uname() output will be like this:

{
  machine = "x86_64",
  release = "10.0.22631",
  sysname = "MINGW32_NT-10.0",
  version = "Windows 11 Pro"
}

sysname will not start with “Windows”, even if you're on Windows. This will break the duplicate prefix feature.

This PR uses a common method to detect Windows.

[akinsho]

LGTM thanks 👍🏾