refactor: prepare the interactive shell in its own module

The prompt-hook step (#41) pushed linux.py past its line count in the
quality baseline. The three steps that make netmiko's fresh shell fit to
read - history off, prompt hooks off, no line wrap - and the prompt
pattern move to napalm_linux/_shell.py, under strict mypy. linux.py
shrinks below where it was; the baseline follows.

tests/test_sudo_off_the_command_line.py

@@ -21,7 +21,7 @@ from unittest.mock import MagicMock, patch
 import pytest
 
 from napalm_linux import LinuxDriver
-from napalm_linux.linux import _HISTORY_OFF
+from napalm_linux._shell import HISTORY_OFF as _HISTORY_OFF
 
 _PW = "s3cr3t 'q' $x"