Setup a basic State enum for the game

[janhohenheim]

Migrated from bevyengine/bevy_github_ci_template#64

Same rationale as #4
#6 is a good candidate for GameState::Boot.

[benfrankel]

As for bikeshedding the name. GameState and AppState are popularly used and familiar, but I would argue that in a game that has multiple state types for different things, they're kinda ambiguous names. What does "GameState" mean? Everything is a game state...

An alternative name would be something like Screen or SequenceState. I use Screen in my own template currently.

I'm totally fine with just going with GameState / AppState though, I don't think it'll cause any confusion for a jam-sized game.

[MiniaczQ]

As mentioned in #3

We can simplify https://github.com/MiniaczQ/bevy-substate-based-ui down to a menu + credits