Add scrollbars to outline panel (#19969)

Part of https://github.com/zed-industries/zed/issues/15324


![image](https://github.com/user-attachments/assets/4f32d585-9bd2-46be-8234-3658a71906ee)

Repeats the approach used in the project panel.

Release Notes:

- Added scrollbars to outline panel

---------

Co-authored-by: Nate Butler <nate@zed.dev>

docs/src/configuring-zed.md

@@ -2271,6 +2271,9 @@ Run the `theme selector: toggle` action in the command palette to see a current
   "auto_fold_dirs": true,
   "indent_guides": {
     "show": "always"
+  },
+  "scrollbar": {
+    "show": null
   }
 }
 ```