Implement new oneshot Vfs for CLI commands and enhance JSON5 serialization

- Added `new_oneshot` method to the Vfs struct, providing a backend suitable for one-time operations without file watching.
- Updated CLI commands (`fmt_project` and `syncback`) to utilize the new oneshot Vfs, improving error handling during file operations.
- Enhanced JSON5 serialization by introducing a custom serializer that directly builds a tree structure, improving performance and maintaining sorted keys.
- Improved error logging in JSON serialization to provide more context on failures.

src/cli/syncback.rs

@@ -69,8 +69,9 @@ impl SyncbackCommand {
             dom_start_timer.elapsed().as_secs_f32()
         );
 
-        let vfs = Vfs::new_default();
-        vfs.set_watch_enabled(false);
+        // Use oneshot Vfs for syncback - file watching isn't needed and
+        // watcher errors shouldn't terminate the process
+        let vfs = Vfs::new_oneshot();
 
         let project_start_timer = Instant::now();
         let session_old = ServeSession::new(vfs, path_old.clone())?;