Fix isolation issue for shared state extension with key path

[lumiasaki]

Delete SharedStateKey and SharedStateValues due to isolation issue. The previous implementation was based on single source of truth, but every variable was setting on or getting from SharedStateKey.currentValue, this might cause problems when you have more than one box and meanwhile you reuse the keys.

Modification is delete SharedStateKey and SharedStateValues completely, every shared state extension owns a separated stateValue, during its initialization phase developer needs to assign your instance of state to shared state extension. Developers do not need to extend SharedStateValues or define your SharedStateKey anymore, just provide an instance of your custom state value.

The rest part is still follow this PR, with same APIs, only one difference is you need to point out type for your key paths as below:

class ViewController: UIViewController, Scene {

    // \.timestamp -> \MyState.timestamp
    @SharedStateInjected(\MyState.timestamp)
    var timestamp: TimeInterval?
}