Create a new `main.zig` for use by `jetzig init` - using the demo app's
`main.zig` is no longer workable due to it depending on auth
middleware, which we do not want to enable by default in new projects.
Remove old bash script for setting up a new project, do everything in
Zig to make it platform agnostic and give us an easy place to add
scaffolding commands in future.
