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

crates/language_tools/src/syntax_tree_view.rs

@@ -110,7 +110,7 @@ impl SyntaxTreeView {
 
         let subscription = cx.subscribe(&editor, |this, _, event, cx| {
             let did_reparse = match event {
-                editor::EditorEvent::Reparsed => true,
+                editor::EditorEvent::Reparsed(_) => true,
                 editor::EditorEvent::SelectionsChanged { .. } => false,
                 _ => return,
             };