Shutdown reticulate finalizer

[dfalbel]

Make sure that exiting the R session will first finalize the reticulate Python session.
See posit-dev/positron#5100 for more details.

[lionel-]

I would avoid using finalizers for this, and also blocking in the finalizer.

Could we shut it down from the positron side via comm msg?
If not, do it from Ark's shutdown message handler, perhaps with some new handler mechanism that comms could register.

[dfalbel]

I'm not really happy with this approach either.

I couldn't find a way to do this from Positron, ideally we need some syncronous callback that is called before the R session exits. Currently positron doesn't have any API that let's us know when the session is about to close.
I briefly talked with @jmcphers about having such callbacks but it seems like a good ammount of work to implement it properly in the Positron side.

I can try attaching attaching this to the Ark shutdown handler, which makes things much more predictable. Although I hope this code is transient and just sits there while we don't have the nice solution implemented in Positron.