Add excerpts into outline panel (#13034)

Follow-up of https://github.com/zed-industries/zed/pull/12637

Adds excerpt items into the outline panel: now all outline items are
initially hidden under excerpt items that could be toggled open/closed
similar to directories.


![Screenshot 2024-06-14 at 10 45
04](https://github.com/zed-industries/zed/assets/2690773/9c9ef91b-1666-43c3-acc4-96f850098a28)

On active editor's selection change, a corresponding outline will be
revealed still, expanding the corresponding excerpt

![Screenshot 2024-06-14 at 10 45
13](https://github.com/zed-industries/zed/assets/2690773/7dfd14f7-4aca-48f2-8760-8e1362b9a043)

Release Notes:

- N/A