Add a license header to a new file in Shady DOM.

Author: bicknellr

packages/shadydom/src/selector-parser.js

@@ -1,3 +1,14 @@
+/**
+ * @license
+ * Copyright (c) 2022 The Polymer Project Authors. All rights reserved. This
+ * code may only be used under the BSD style license found at
+ * http://polymer.github.io/LICENSE.txt The complete set of authors may be found
+ * at http://polymer.github.io/AUTHORS.txt The complete set of contributors may
+ * be found at http://polymer.github.io/CONTRIBUTORS.txt Code distributed by
+ * Google as part of the polymer project is also subject to an additional IP
+ * rights grant found at http://polymer.github.io/PATENTS.txt
+ */
+
 /**
  * A map describing parentheses-like grouping characters and their behavior. The
  * keys are the group's start character and the value's `end` property contains