feat: improve dev environment runner

[jbranchaud]

this addresses two issues:

1. if a process like the stripe-check script failed, the error was scrolling off the screen and the rest of the dev server appeared to be running fine even though the stripe webhook listener wasn't going to work. Adding --kill-others-on-fail to the concurrently call means the dev server will bail early so that I can see and address whatever issue came up.
2. the react-email part of the dev server isn't needed for typical development, so I'm disabling it by default and you can opt-in to using it with the WITH_EMAIL env var. Instructions in README.