Exit R gracefully exits the Reticulate Python session

[dfalbel]

Addresses #5083
Requires Ark side: posit-dev/ark#603
This is built on top of #5056 so should merge it before this one.

In the Ark side we added a finalizer using reg.finalizer to the reticulate namespace. So whenever the reticulate namespace is unloaded (or when the R session is about to exit), we'll send a shutdown event to the reticulate extension. The R side waits for the event to be processed and for the comm client to be disposed before really finalizing the session. Thus making sure that the reticulate Python session has gracefully exited before ending the R session - which would cause IPykernel to crash otherwise.

I'm not sure this is the best approach although it's the one that doesn't need to introduce any new concepts.
We could also:

1. Add API's to register shutdown callbacks to positron.RuntimeSession, these callbacks would be await before calling the session shutdown routines.

2. Perhaps exposing the Exiting RuntimeState. Although this might not be enough, because we really need to wait for the Python session to end before proceeding.

   positron/src/vs/workbench/services/languageRuntime/common/languageRuntimeService.ts

   Lines 299 to 301 in bf3de59

   	/** The runtime is in the process of shutting down. */
   	Exiting = 'exiting',

3. Introducing the concept of parent/child sessions and 'parents' would be resposnible for shutting down their child sessions before shutting them down.

@jmcphers do you have a sense if the current approach is reasonable?