Improve layer typings

src/ol/layer/VectorImage.js

@@ -51,6 +51,8 @@ import {assign} from '../obj.js';
  * property on the layer object; for example, setting `title: 'My Title'` in the
  * options means that `title` is observable, and has get/set accessors.
  *
+ * @template {import("../source/Vector.js").default} VectorSourceType
+ * @extends {BaseVectorLayer<VectorSourceType>}
  * @api
  */
 class VectorImageLayer extends BaseVectorLayer {