Observe outside changes to AppStorage values

[kaascevich]

The title says it all.

Backend-agnostic (and thus works on all backends just realized that UserDefaults does not feature KVO on non-Apple platforms, I’ll have to figure something else out over there).

• Adds a new requirement to AppStorageProvider:

  func listenToChanges(_ action: @MainActor @Sendable @escaping (String) -> Void)

[kaascevich]

If I can get it to work, I may update this so that each individual AppStorage instance registers a listener instead of only one being registered on app startup. That way we won’t have to refresh the entire scene graph every time an external change occurs.