=head2 method print-nl

    method print-nl(IO::Handle:D: --> True)

Writes a newline to the filehandle. The newline marker defaults
to C<\n> unless another marker has been specified in the call to
L<open>.

    my $fh = open 'path/to/file', :w, nl-out => "\r\n";
    $fh.print("some text");
    $fh.print-nl;                                        # \r\n
    $fh.close;