Fully halt background scanner threads when dropping Worktree

* Rework fsevent API to expose a handle for halting the event stream

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

fsevent/examples/events.rs

@@ -5,12 +5,12 @@ fn main() {
     let paths = args().skip(1).collect::<Vec<_>>();
     let paths = paths.iter().map(Path::new).collect::<Vec<_>>();
     assert!(paths.len() > 0, "Must pass 1 or more paths as arguments");
-    let stream = EventStream::new(&paths, Duration::from_millis(100), |events| {
+    let (stream, _handle) = EventStream::new(&paths, Duration::from_millis(100));
+    stream.run(|events| {
         eprintln!("event batch");
         for event in events {
             eprintln!("  {:?}", event);
         }
         true
     });
-    stream.run();
 }