outline panel: Add indent guides (#19719)

See #12673

| File | Search |
|--------|--------|
| <img width="302" alt="image"
src="https://github.com/user-attachments/assets/44b8d5f9-8446-41b5-8c0f-e438050f0ac9">
| <img width="301" alt="image"
src="https://github.com/user-attachments/assets/a2e6f77b-6d3b-4f1c-8fcb-16bd35274807">
|



Release Notes:

- Added indent guides to the outline panel

docs/src/configuring-zed.md

@@ -2237,6 +2237,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
   "folder_icons": true,
   "git_status": true,
   "indent_size": 20,
+  "indent_guides": true,
   "auto_reveal_entries": true,
   "auto_fold_dirs": true,
 }