chore(base-controller): Restore subscrbe property

The `subscribe` property was removed in #3590 because it is no longer
used. It has been restored here so that we can release the
`@metamask/base-controller` without any breaking changes.

This property's description has been updated to clarify that it's no
longer used, and it's now marked as deprecated.

packages/base-controller/src/BaseControllerV2.ts

@@ -113,6 +113,17 @@ export class BaseController<
 
   public readonly metadata: StateMetadata<ControllerState>;
 
+  /**
+   * The existence of the `subscribe` property is how the ComposableController used to detect
+   * whether a controller extends the old BaseController or the new one. We set it to `undefined` here to
+   * ensure the ComposableController never mistakes them for an older style controller.
+   *
+   * This property is no longer used, and will be removed in a future release.
+   *
+   * @deprecated This will be removed in a future release
+   */
+  public readonly subscribe: undefined;
+
   /**
    * Creates a BaseController instance.
    *