Improve running systems declaration about sim-ecs

[minecrawler]

@iamcsharper I changed the way systems are assigned to states. It now works as per your request. The changes are only in the master branch, but will be included in the next release 😉

// State definition
class GameState extends State {
    activate() {
        console.log('Game State is active, now!');
    }
    
    deactivate() {
        console.log('Game State is inactive, now!');
    }
}

// System definition
class GameSystem extends System {
    readonly query = new Query({ /* ... */ });
    readonly states = [GameState];

    run() { /* ... */ }
}

// Dispatch a state once
world.dispatch(GameState);

// OR start running in a state
world.run({ initialState: GameState });

By the way, if you want to run all systems, you can do so by omitting the initial state, or if you want to switch from a state to running all systems, you can simply pass State as state. If you created the world using buildWorld(), the state State was automatically populated with all registered systems.

[minecrawler]

As soon as you reach 50+ systems it gets really annoying to keep your eye on both GameState and createWorld() function

Yes, and it's a problem which directly conflicts with the use-case of sim-ecs. There is a work-item in the "New Features" project, which waits for the bevy_ecs centralized_run_criteria RFC. I am aware of the shortcomings of the current situation, however I want to derive some of the ideas from bevy to implement a solid solution. Once they move forward, I want to take a look at how to make run criteria (as in which systems should run when, including the state) easy to set up.

using decorators

Decorators are a boon, however they are still in draft and experimental, which is why I refrained from adding them. Once they are finalized and not behind a feature-flag anymore, I will probably add some to remove boilerplate where possible.

or simply a readonly array

Since there are fewer states than systems, this could be a short-term fix. I will look into it for 0.4!

[vmwxiong]

For what it's worth, decorators have been around for a fair bit now, and I have seen them used effectively already. I know it's not ideal, but I'm guessing that if changes are made they will be minor, and given the state of this project, it might be worth integrating now and just dealing with the change if and when it comes. Might help clarify some design decisions along the way, just my two cents.