Make locally-built ghstack easier to use

After the migration to Poetry (#32), it's a bit more difficult to use ghstack (e.g. for experimentation, to check or validate behaviors) while locally developing it. The solution given in the README is to use poetry shell, but that is a bit unergonomic because it requires one to cd into their clone of ghstack, run poetry shell, then cd out again.

One alternative could be to put a shell script like this on your PATH (assuming your clone of ghstack is in ~/github/ezyang/ghstack):

#!/usr/bin/env bash
$(cd ~/github/ezyang/ghstack && poetry run which ghstack) "$@"

The downside of this is of course that it has a hardcoded path. Maybe it would be worth it to add a Makefile to this repo such that make install automatically puts such a file on the PATH, using the actual path to the repo clone for the cd subshell command.

Start with the README's current Poetry shell instructions and inspect the repository's Poetry configuration. Define the install workflow around the proposed Makefile and PATH wrapper, then verify that a locally built ghstack can be invoked without changing directories.