Extend mediaWiki.confirmCloseWindow to trigger it manually

Some extensions and/or functions doesn't reload the page to destroy a form
that can contain some user input. To use confirmCloseWindow to cover these forms,
too, we need a way to manually trigger this module and show the warning
to the user.

Bug: T94194
Change-Id: I897330bc62b9ef3bfd04f61f5c5d0744c8ec61d3