Shut down supervisor gracefully on SIGINT/SIGTERM (#8011)

Speculative fix for #7991. I could not reproduce with the steps in that
issue, but was able to reproduce a similar situation synthetically as
follows:

- start with Positron with supervisor's idle shutdown timeout set to 4
hours
- start R and Python, observer `kcserver` is running as part of the
Positron process tree
- close Positron, observe `kcserver` is now owned by `init(1)` and still
running (expected)
- `killall kcserver` to terminate the supervisor
- now `ark` is running all by its lonesome, unsupervised (unexpected)

To help prevent this from happening, this change adds signal handlers to
the supervisor that cause it to send shutdown requests to the kernels it
is running before it exits itself.

QA team, could you review by seeing if this fixes the issue for you?

Author: jmcphers

extensions/positron-supervisor/package.json

@@ -150,7 +150,7 @@
   },
   "positron": {
     "binaryDependencies": {
-      "kallichore": "0.1.43"
+      "kallichore": "0.1.45"
     }
   },
   "extensionDependencies": [