Zed nightly: fs: Make `OsWatcher` own a `BackgroundExecutor` (#65700)

Summary

While looking into something else with the extension CLI, I noticed that
sometimes the CLI would not shut down after compilation.

GPTs analysis was the following:

A macOS process sample showed this sequence:

1. main finishes writing archive.tar.gz and manifest.json.
2. Dropping RealFs drops its fields in declaration order.
3. Dropping a watcher’s channel wakes its dispatch task, but the
   scheduler is already gone.
4. That wake synchronously drops the task’s channel receiver while the
   event-listener notification lock is held, deadlocking.

Thus, fixing this here by having the watcher own an executor which
prevents this from happening. Not the best fix, but better than having
this around.

Testing

Manually tested as well as with a test below.

Self-review

• I've reviewed my diff for quality, security, reliability, and
  performance.
• [-] UI changes follow the
  checklist.
• Tests cover the new or changed behavior.

Release Notes:

• N/A