Restore ComposableController compatibility between different BaseCont…

…rollers (#458)

When adding support for BaseControllerV2 to the ComposableController
in #447, we accidentally removed support for using two different
versions of the BaseController in the same ComposableController. This
is because we used the `instanceof` operator to check whether a
controller was extended from BaseController.

The `instanceof` check has been replaced by a check for the `subscribe`
function. It seems unlikely that a BaseControllerV2 controller will
ever have a `subscribe` function because the controller messenger
will be used for all events.

The BaseControllerV2 class now prevents the use of the `subscribe`
property, ensuring the duck typing strategy used by the
ComposableController to tell old and new controllers apart continues to
work.

src/BaseControllerV2.ts

@@ -131,6 +131,14 @@ export class BaseController<
 
   public readonly metadata: StateMetadata<S>;
 
+  /**
+   * The existence of the `subscribe` property is how the ComposableController detects whether a
+   * controller extends the old BaseController or the new one. We set it to `never` here to ensure
+   * this property is never used for new BaseController-based controllers, to ensure the
+   * ComposableController never mistakes them for an older style controller.
+   */
+  public readonly subscribe: never;
+
   /**
    * Creates a BaseController instance.
    *

src/ComposableController.ts

@@ -62,8 +62,8 @@ export class ComposableController extends BaseController<never, any> {
     this.messagingSystem = messenger;
     this.controllers.forEach((controller) => {
       const { name } = controller;
-      if (controller instanceof BaseController) {
-        controller.subscribe((state) => {
+      if ((controller as BaseController<any, any>).subscribe !== undefined) {
+        (controller as BaseController<any, any>).subscribe((state) => {
           this.update({ [name]: state });
         });
       } else if (this.messagingSystem) {