Return `this` from `BatchedMesh#copy` (#27161)

Makes it consistent with `Mesh#copy` and `Object3D#copy`

examples/jsm/objects/BatchedMesh.js

@@ -785,6 +785,8 @@ class BatchedMesh extends Mesh {
 		this._matricesTexture = source._matricesTexture.clone();
 		this._matricesTexture.image.data = this._matricesTexture.image.slice();
 
+		return this;
+
 	}
 
 	dispose() {