* Allow injecting some initial output to new panes
* Have the update checker set this new-pane-banner to a short
upsell to let the user know there is an update.
* Refactor toast notifications into their own crate
* Have the update checker call a new stub function that triggers
a toast notification with an URL... but it does nothing because
the rust ecosystem doesn't support this on macos yet and I'm
writing this code there
