cleanup: Mark globals used from other modules as exported

eslint cannot figure out that those symbols are used from other modules
via imports, so they trigger unused-variable errors. To fix, explicitly
mark those symbols as exported.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/627

js/portalHelper/main.js

@@ -1,3 +1,4 @@
+/* exported main */
 const Format = imports.format;
 const Gettext = imports.gettext;
 const { Gio, GLib, GObject, Gtk, Pango, Soup, WebKit2: WebKit } = imports.gi;