[Feature Request] Support for page unload detection in sandboxed mode

Hey AMP/worker-dom team,

At Permutive, we've been using the `sandboxed` mode of amp-script for our script. The change to support this in worker-dom was introduced in #1042

One request we had and that I was discussing with @samouri is the ability to detect a page unload event.

Right now, in sandboxed mode we cannot attach any event listeners, and this is understandable based on the restrictions that the `sandboxed` mode places on scripts running within it.

However, I was wondering if we can allow an event listener to be attached for a page unload event. Based on our cross device testing we currently use the `pagehide` event on desktop and `visibilitychange` for mobile.

I was wondering if we can get access to both while our script is running in sandboxed mode?

### Use Case

In response to a page unload event, we wish to be able to send a beacon request with some engagement related metrics from the page.

Thanks!

Start by reviewing the sandboxed-mode change introduced in #1042 and the current restrictions on event listeners. Trace how pagehide and visibilitychange are handled, then define completion as allowing both events in sandboxed mode for the stated page-unload beacon use case.

Written by the indexing model from the issue text.