afterUnloadDocument hook

[georeith]

This PR adds an afterUnloadDocument hook that runs whenever a previously loaded document is removed by the server, for any reason.

The motivation for adding this feature is because I monitor documents being opened and closed in a load balancer that speaks to an extension and I noticed the load balancer was not removing all documents for some reason when I listen to it like this:

{
  async afterLoadDocument({
    documentName,
  }: onLoadDocumentPayload): Promise<void> {
   addToLoadBalancer(documentName);
  }

  async onDisconnect({
    documentName,
    clientsCount,
  }: onDisconnectPayload): Promise<void> {
    if (clientsCount === 0) {
      removeFromLoadBalancer(documentName);
    }
  }
}

I also fixed up the type of the hooks function so that you can no longer pass an incorrect payload that does not match the exact one for that hook specified in the types. There were a couple of cases where they didn't match and I had to fix those. This was to prevent issues such as #641 in the future.

[georeith]

@janthurau just bringing to your attention.

[janthurau]

Thanks a lot for this!! :-)