Description

This PR tries to correct the problem of nushell scripts being made executable on Windows systems. In order to do this, these steps need to take place.

1. assoc .nu=nuscript
2. ftype nuscript=C:\path\to\nu.exe '%1' %*
3. modify the env var PATHEXT by appending ;.NU at the end
    
   Once those steps are done and this PR is landed, one should be able to create a script such as this.

❯ open im_exe.nu
def main [arg] {
  print $"Hello ($arg)!"
}

Then they should be able to do this to run the nushell script.

❯ im_exe Nushell
Hello Nushell!

Under-the-hood, nushell is shelling out to cmd.exe in order to run the nushell script.

User-Facing Changes

closes #13020

Tests + Formatting

After Submitting