feat: scaffold an app without a default module

[lumtis]

Some projects, notably serious projects, don't need a default module to be scaffolded. For example, spn doesn't need a module spn. Having this feature could be a time saver because removing the default module from the app requires to remove many lines in app.go file

The solution could be to provide a flag for the app command like starport app ... --no-default-module

[fadeev]

Very sensible proposal 👍