feat: adapter-node shutdown event

[karimfromjordan]

This is a small improvement and the missing piece to the recent graceful shutdown feature.
After the server has closed gracefully Node will emit the exit event. As others have pointed out however if you have something running in the background the app will continue to run and exit will never be emitted. The exit event isn't ideal anyway because it will close your app as soon as possible so we can't do any custom async cleanup work in a process.on('exit', () => { ... }) handler. That means calling process.exit() to manually emit the exit event isn't an option. It defeats the point of a graceful shutdown. This PR emits a custom event instead that users of adapter-node can listen to and use to perform any kind of cleanup work right after the HTTP server has closed.

process.on('sveltekit:shutdown', async (reason) => {
  await db.close();
  await jobs.stop();
  process.exit();
});

We briefly discussed the option of exporting a function to register a hook instead but with the way adapters work this doesn't seem to be possible today.

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