fix: use strict mode, not sloppy mode

This could theoretically change behavior for `{{flutter_js}}`, but I think it's fine.

Author: lishaduck

__brick__/web/flutter_bootstrap.js

@@ -1,3 +1,4 @@
+"use strict";
 {{=<% %>=}}{{flutter_js}}<%={{ }}=%>
 {{=<% %>=}}{{flutter_build_config}}<%={{ }}=%>