Add SwiftUI App Lifecycle

[carson-katri]

This PR adds the App protocol, as well as Scenes, modifiers, and changes to the renderer to make building apps entirely with Tokamak simpler:

struct MyApp: App {
  var body: some Scene {
    WindowGroup("Tokamak") {
      Text("Hello, world!")
    }
  }
}

MyApp.main()

I've also implemented @AppStorage and @SceneStorage with the ability for renderers to override the default containers:

@AppStorage("count") var count: Int = 0 // Persists/updates across tabs.
@SceneStorage("count") var sceneCount: Int = 0  // Persists across single-tab reloads.

AppStorage will automatically update all tabs when one changes the value. SceneStorage will automatically be loaded on page-load as well as when any View in the app updates it. TokamakDOM uses localStorage and sessionStorage.

This PR also contains the ScenePhase environment value:

@Environment(\.scenePhase) var phase: ScenePhase

• .active: Tab is active in the window of the browser.
• .background: Tab is open, but not active.

This automatically updates with the PageVisibility API.

[j-f1]

Another cue to look for here is document.hasFocus() — if it’s false but document.visiblityState is visible, the page is .inactive. I couldn’t find out if there were any events that trigger when this changes, though.

[carson-katri]

Doesn't seem like it looking through MDN. If you find something lmk.

[MaxDesiatov]

Turns out this requires latest carton to work (because of JavaScriptKit runtime fix), which kind of pushes me towards releasing Tokamak 0.2.0 and carton 0.4.0 without this PR...

[MaxDesiatov]

Is there anything in the lifecycle API that isn't covered by this PR? I'm just wondering if this should be added to the progress document

[carson-katri]

Custom scenes are not yet supported. We could support these in a future PR IMO:

struct TokamakDemoScene : Scene {
  var body: some Scene {
    WindowGroup { ... }
  }
}

[MaxDesiatov]

Fantastic! 👏