feat(react-email): Build preview app and run it built instead of as dev

[gabrielmfern]

This PR changes it so that the email dev command runs the preview app as in production in a built format. This makes it so that it is much faster, it also irons out the different issues that arise from this new way to run the preview app.

This PR still keeps the published source code for the preview app with the purpose of copying and building it with email build still.

One more thing this PR does is add a bit of feedback to the user when running email dev that I think is particularly prettier.