Use strict mode

Author: piroor

webextensions/background/index.js

@@ -3,6 +3,7 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */
+'use strict';
 
 import {
   log,

webextensions/sidebar/index.js

@@ -3,6 +3,8 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */
+'use strict';
+
 import '/extlib/l10n.js';
 
 import {