Add stub callback for notify_server_did_finish_handling_input_event

[AtkinsSJ]

This doesn't need to do anything yet, but will do once we start passing events to web content before they're passed to our GUI.

This implements the changes from https://github.com/SerenityOS/serenity/pull/16128 - please merge them together. :^)

[AtkinsSJ]

I was briefly worried about the CI failure, then I remembered, derp, of course it fails, it refers to things in the Serenity PR, that's the whole point! 😅