Add zsh install script

run_once_before_11-set-zsh.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -euo pipefail
+
+ZSH_PATH="$(which zsh)"
+
+if !(grep -q "^$ZSH_PATH$" /etc/shells;) then
+  echo "$ZSH_PATH" | sudo tee -a /etc/shells > /dev/null
+  echo "Added $ZSH_PATH to /etc/shells"
+fi
+
+if [[ "$(getent passwd $USER | cut -d: -f7)" != "$ZSH_PATH" ]]; then
+  chsh -s "$ZSH_PATH"
+  echo "Set ZSH as default, log out and log back in (or reboot) for the change to take effect"
+else
+    echo "Default shell is already ZSH"
+fi