commands: move shutdown command to pmb/commands (MR 2252)

Another one bites the dust

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/commands/shutdown.py

@@ -0,0 +1,14 @@
+# Copyright 2024 Caleb Connolly
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+from __future__ import annotations
+from pmb import commands
+import pmb.chroot
+
+class Shutdown(commands.Command):
+    def __init__(self):
+        pass
+
+    def run(self):
+        pmb.chroot.shutdown()
+