Reduce amount of foreground tasks spawned on multibuffer/editor updates (#41479)

When doing a project wide search in zed on windows for `hang`, zed
starts to freeze for a couple seconds ultimately starting to error with
`Not enough quota is available to process this command.` when
dispatching windows messages. The cause for this is that we simply
overload the windows message pump due to the sheer amount of foreground
tasks we spawn when we populate the project search.

This PR is an attempt at reducing this.

Release Notes:

- Reduced hangs and stutters in large project file searches

crates/worktree/src/worktree.rs

@@ -1318,7 +1318,8 @@ impl LocalWorktree {
         let entry = self.refresh_entry(path.clone(), None, cx);
         let is_private = self.is_path_private(path.as_ref());
 
-        cx.spawn(async move |this, _cx| {
+        let this = cx.weak_entity();
+        cx.background_spawn(async move {
             // WARN: Temporary workaround for #27283.
             //       We are not efficient with our memory usage per file, and use in excess of 64GB for a 10GB file
             //       Therefore, as a temporary workaround to prevent system freezes, we just bail before opening a file
@@ -1702,6 +1703,7 @@ impl LocalWorktree {
         };
         let t0 = Instant::now();
         let mut refresh = self.refresh_entries_for_paths(paths);
+        // todo(lw): Hot foreground spawn
         cx.spawn(async move |this, cx| {
             refresh.recv().await;
             log::trace!("refreshed entry {path:?} in {:?}", t0.elapsed());