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/quality_baseline.json

@@ -22,7 +22,7 @@
   "napalm_linux.linux"
  ],
  "python_file_lines": {
-  "napalm_linux/linux.py": 2307
+  "napalm_linux/linux.py": 2288
  },
  "untyped_public": []
 }