feat(cli): add bin/doomscript

Meant as a simple elisp interpreter with Doom's CLI framework preloaded.
Can be used as a shebang line:

  #!/usr/bin/env doomscript
  (princ "hello world!")

This isn't used for bin/doom because it requires doomscript be in your
$PATH, and any attempt to resolve its location in bin/doom's shebang
line would reduce its portability. Neither of these should be an issue
for the type of user who'd find this useful.

bin/doomscript

@@ -0,0 +1,61 @@
+#!/usr/bin/env sh
+# This is a shebang interpreter for launching emacs lisp scripts with Doom's CLI
+# framework preloaded and all the metadata it needs initialized. Use it like so:
+#
+#   #!/usr/bin/env doomscript
+#   (print! "Hello world!")
+#
+# For this to work, this file must be in your $PATH.
+#
+#   export PATH="$HOME/.emacs.d/bin:$PATH"
+#
+# This can also be exploited to evaluate arbitrary elisp against Doom's CLI
+# environment.
+#
+# This isn't used for bin/doom because of the $PATH requirement (and using
+# $BASH_SOURCE to locate it would reduce its POSIX compliance). This shouldn't
+# be an issue for folks writing their own CLIs, however.
+
+set -e
+case "$EMACS" in
+    *term*) EMACS=emacs ;;
+    *) EMACS="${EMACS:-emacs}" ;;
+esac
+
+TMPDIR=${TMPDIR:-`$EMACS -Q --batch --eval '(princ (temporary-file-directory))' 2>/dev/null`}
+if [ -z "$TMPDIR" ]; then
+    >&2 echo "Error: failed to run Emacs with command '$EMACS'"
+    >&2 echo
+    >&2 echo "Are you sure Emacs is installed and in your \$PATH?"
+    exit 1
+fi
+
+if [ -z "$EMACSDIR" ]; then
+    EMACSDIR="${XDG_CONFIG_HOME:-$HOME/.config}/emacs"
+    [ -d "$EMACSDIR" ] || EMACSDIR="$HOME/.emacs.d"
+    if [ ! -d "$EMACSDIR" ]; then
+        >&2 echo "Failed to find \$EMACSDIR in ~/.config/emacs or ~/.emacs.d"
+        exit 1
+    fi
+fi
+
+export __DOOMPID="${__DOOMPID:-$$}"
+export __DOOMSTEP="$((__DOOMSTEP+1))"
+export __DOOMGEOM="${__DOOMGEOM:-`tput cols lines 2>/dev/null`}"
+export __DOOMGPIPE=${__DOOMGPIPE:-$__DOOMPIPE}
+export __DOOMPIPE=; [ -t 0 ] || __DOOMPIPE+=0; [ -t 1 ] || __DOOMPIPE+=1
+
+tmpfile="$TMPDIR/doomscript.${__DOOMPID}"
+
+target="$1"
+shift
+$EMACS -Q --batch \
+    --load "$EMACSDIR/core/core-cli" \
+    --load "$target" \
+    -- "$@" || exit=$?
+# Execute exit-script, if requested (to simulate execve)
+if [ "${exit:-0}" -eq 254 ]; then
+    sh "${tmpdir}/doom.${__DOOMPID}.${__DOOMSTEP}.sh" "$0" "$@" && true
+    exit="$?"
+fi
+exit $exit