Add install script for GitHub codespaces

See https://github.com/twpayne/dotfiles/blob/master/install.sh

install.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+# Adapted from twpayne's (chezmoi author) install script
+# https://github.com/twpayne/dotfiles/blob/master/install.sh
+
+set -euo pipefail
+
+if [ ! "$(command -v chezmoi)" ]; then
+  bin_dir="$HOME/.local/bin"
+  chezmoi="$bin_dir/chezmoi"
+  if [ "$(command -v curl)" ]; then
+    sh -c "$(curl -fsSL https://git.io/chezmoi)" -- -b "$bin_dir"
+  elif [ "$(command -v wget)" ]; then
+    sh -c "$(wget -qO- https://git.io/chezmoi)" -- -b "$bin_dir"
+  else
+    echo "To install chezmoi, you must have curl or wget installed." >&2
+    exit 1
+  fi
+else
+  chezmoi=chezmoi
+fi
+
+# POSIX way to get script's dir: https://stackoverflow.com/a/29834779/12156188
+script_dir="$(cd -P -- "$(dirname -- "$(command -v -- "$0")")" && pwd -P)"
+# exec: replace current process with chezmoi init
+exec "$chezmoi" init --apply "--source=$script_dir"