Refactor plugins; Highlight debug-assertions in Cargo.toml

[wanderrful]

Hi there,

I found it more useful to isolate all of the starter, default plugins, logic, etc into its own "default" folder, so that I can better decouple the bevy_game starter template from the additional things I want to add to the project.

Also, incidentally it shows users of this template how they can organize their own code as well.

Lastly, I added a debug-assertions line in the Cargo.toml so that it's more obvious to users of this template how they can disable the default "frame diagnostic" logging that would otherwise normally flood stdout whenever they run the application.

edit - Also, by using public and private, it sets a good example for the users of this template to follow, so that their code is more organized and not just 5_000 lines in a single file.