Improve layer typings

src/ol/layer/BaseTile.js

@@ -42,7 +42,8 @@ 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.
  *
- * @extends {Layer<import("../source/Tile.js").default>}
+ * @template {import("../source/Tile.js").default} TileSourceType
+ * @extends {Layer<TileSourceType>}
  * @api
  */
 class BaseTileLayer extends Layer {