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/multi_buffer/src/multi_buffer.rs

@@ -94,9 +94,9 @@ pub enum Event {
     DiffUpdated {
         buffer: Model<Buffer>,
     },
-    LanguageChanged,
+    LanguageChanged(BufferId),
     CapabilityChanged,
-    Reparsed,
+    Reparsed(BufferId),
     Saved,
     FileHandleChanged,
     Closed,
@@ -538,9 +538,13 @@ impl MultiBuffer {
         });
 
         if let Some(buffer) = self.as_singleton() {
-            return buffer.update(cx, |buffer, cx| {
+            buffer.update(cx, |buffer, cx| {
                 buffer.edit(edits, autoindent_mode, cx);
             });
+            cx.emit(Event::ExcerptsEdited {
+                ids: self.excerpt_ids(),
+            });
+            return;
         }
 
         let original_indent_columns = match &mut autoindent_mode {
@@ -1639,8 +1643,8 @@ impl MultiBuffer {
             language::Event::Reloaded => Event::Reloaded,
             language::Event::DiffBaseChanged => Event::DiffBaseChanged,
             language::Event::DiffUpdated => Event::DiffUpdated { buffer },
-            language::Event::LanguageChanged => Event::LanguageChanged,
-            language::Event::Reparsed => Event::Reparsed,
+            language::Event::LanguageChanged => Event::LanguageChanged(buffer.read(cx).remote_id()),
+            language::Event::Reparsed => Event::Reparsed(buffer.read(cx).remote_id()),
             language::Event::DiagnosticsUpdated => Event::DiagnosticsUpdated,
             language::Event::Closed => Event::Closed,
             language::Event::CapabilityChanged => {