Fire the pageswap event when navigating away from a document

See also w3c/csswg-drafts#9819.

The pageswap event is specified here to work as follows:

* It's sent whenever a document navigates away, to a different document, but not when unloaded for other reasons, such as closing the browser types or unloading an ancestor.

* It includes a NavigationActivation object for same-origin navigations.

* If the navigation might trigger a cross-document view-transition (this is determined when snapshotting the source document, by reading the CSS), pageswap is fired and then unloading is deferred until the old state is captured for the view transition. Otherwise, if the view transition is skipped, unloading happens immediately.

Closes #9702.