js: Add "use strict" directive to CommonJS files.

ES and TypeScript modules are strict by default and don’t need this
directive.  ESLint will remind us to add it to new CommonJS files and
remove it from ES and TypeScript modules.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

frontend_tests/node_tests/keydown_util.js

@@ -1,3 +1,5 @@
+"use strict";
+
 set_global("$", global.make_zjquery());
 
 zrequire("keydown_util");