Add explicit strict mode in components index file

If this file gets compiled with terser via sprockets
it complain about top-level await is not available.

This code will always ever run in context of a module,
which implicitely implies strict mode, but Terser need
this hint anyway.

Refs ahorek/terser-ruby#62

Closes #3245

(cherry picked from commit 1c19cd4)

Author: tvdeyen

app/javascript/alchemy_admin/components/index.js

@@ -1,3 +1,6 @@
+"use strict"
+// ^ add support for top-level await in Terser
+
 import "alchemy_admin/components/action"
 import "alchemy_admin/components/attachment_select"
 import "alchemy_admin/components/button"