Add two built-in events to PortRPC which are dispatched when PortRPC connects to
a port and when it is destroyed or the containing frame is unloaded. These
events can be used in the counterpart PortRPC to confirm that the sender has
successfully received and connected to the port, and to get notified when the
port goes away.

Sending the "close" event in the context of a window unloading in Safari
<= 15 requires a workaround that involves registering a handler in the
parent frame. This handler is currently installed only in the host frame.

The new "close" event is used to replace the "frameDestroyed" message
that was used by guests to notify the sidebar when it went away. This solves
several problems:

 - It centralizes the workaround for https://bugs.webkit.org/show_bug.cgi?id=231167
   in `post-rpc.js`, instead of having it spread between several
   modules.

 - It provides a way to tear down the guest-host connection when the
   guest goes away.

 - It provides a way to handle the case where a guest is unloaded before it
   has received and connected to the port, by having the host/sidebar frames
   expect the "connect" call within a timeout. This is not yet implemented.