Require ObservableScopes of state holders to be specified explicitly

Previously we had a ViewModel class which was responsible for little more than creating an ObservableScope. However, since this ObservableScope would be created implicitly upon view model construction, it became a tad bit harder for callers to remember to eventually end the scope (as you wouldn't just have to remember to end ObservableScopes, but also to destroy ViewModels). Requiring the scope to be specified explicitly by the caller also makes it possible for the caller to reuse the scope for other purposes, reducing the number of scopes mentally in flight that need tending to, and for all state holders (not just view models) to be handled uniformly by helper functions such as generateKeyed$.

src/state/AppViewModel.ts

@@ -6,14 +6,16 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { MediaDevices } from "./MediaDevices";
-import { ViewModel } from "./ViewModel";
+import { type ObservableScope } from "./ObservableScope";
 
 /**
  * The top-level state holder for the application.
  */
-export class AppViewModel extends ViewModel {
+export class AppViewModel {
   public readonly mediaDevices = new MediaDevices(this.scope);
 
   // TODO: Move more application logic here. The CallViewModel, at the very
   // least, ought to be accessible from this object.
+
+  public constructor(private readonly scope: ObservableScope) {}
 }