send a dispose command before killing the child process

[brettfo]

When stopping/restarting the backing process from the VS Code extension, we blindly call .kill(), but some kernels, particularly remote proxies, might need to clean up some resources. We should first send DisposeCommand or similar, give the backing process a short amount of time to respond, and only then would we manually send the .kill() signal if it didn't gracefully exit on its own.

[jonsequitur]

The child process should be able to handle its own disposal on termination rather than needing a custom gesture for it. Is there a specific case where this is not happening?