Introduce an outline panel (#12637)

Adds a new panel: `OutlinePanel` which looks very close to project
panel:

<img width="256" alt="Screenshot 2024-06-10 at 23 19 05"
src="https://github.com/zed-industries/zed/assets/2690773/c66e6e78-44ec-4de8-8d60-43238bb09ae9">

has similar settings and keymap (actions work in the `OutlinePanel`
context and are under `outline_panel::` namespace), with two notable
differences:
* no "edit" actions such as cut/copy/paste/delete/etc.
* directory auto folding is enabled by default

Empty view: 
<img width="841" alt="Screenshot 2024-06-10 at 23 19 11"
src="https://github.com/zed-industries/zed/assets/2690773/dc8bf37c-5a70-4fd5-9b57-76271eb7a40c">


When editor gets active, the panel displays all related files in a tree
(similar to what the project panel does) and all related excerpts'
outlines under each file.
Same as in the project panel, directories can be expanded or collapsed,
unfolded or folded; clicking file entries or outlines scrolls the buffer
to the corresponding excerpt; changing editor's selection reveals the
corresponding outline in the panel.

The panel is applicable to any singleton buffer:
<img width="1215" alt="Screenshot 2024-06-10 at 23 19 35"
src="https://github.com/zed-industries/zed/assets/2690773/a087631f-5c2d-4d4d-ae25-30ab9731d528">

<img width="1728" alt="image"
src="https://github.com/zed-industries/zed/assets/2690773/e4f8082c-d12d-4473-8500-e8fd1051285b">

or any multi buffer:

(search multi buffer)

<img width="1728" alt="Screenshot 2024-06-10 at 23 19 41"
src="https://github.com/zed-industries/zed/assets/2690773/60f768a3-6716-4520-9b13-42da8fd15f50">

(diagnostics multi buffer)
<img width="1728" alt="image"
src="https://github.com/zed-industries/zed/assets/2690773/64e285bd-9530-4bf2-8f1f-10ee5596067c">

Release Notes:
- Added an outline panel to show a "map" of the active editor

crates/editor/src/editor.rs

@@ -149,6 +149,9 @@ use workspace::{OpenInTerminal, OpenTerminal, TabBarSettings, Toast};
 
 use crate::hover_links::find_url;
 
+pub const FILE_HEADER_HEIGHT: u8 = 1;
+pub const MULTI_BUFFER_EXCERPT_HEADER_HEIGHT: u8 = 1;
+pub const MULTI_BUFFER_EXCERPT_FOOTER_HEIGHT: u8 = 1;
 pub const DEFAULT_MULTIBUFFER_CONTEXT: u32 = 2;
 const CURSOR_BLINK_INTERVAL: Duration = Duration::from_millis(500);
 const MAX_LINE_LEN: usize = 1024;
@@ -529,6 +532,7 @@ pub struct Editor {
     tasks: BTreeMap<(BufferId, BufferRow), RunnableTasks>,
     tasks_update_task: Option<Task<()>>,
     previous_search_ranges: Option<Arc<[Range<Anchor>]>>,
+    file_header_size: u8,
 }
 
 #[derive(Clone)]
@@ -1651,9 +1655,8 @@ impl Editor {
             }),
             merge_adjacent: true,
         };
+        let file_header_size = if show_excerpt_controls { 3 } else { 2 };
         let display_map = cx.new_model(|cx| {
-            let file_header_size = if show_excerpt_controls { 3 } else { 2 };
-
             DisplayMap::new(
                 buffer.clone(),
                 style.font(),
@@ -1661,8 +1664,8 @@ impl Editor {
                 None,
                 show_excerpt_controls,
                 file_header_size,
-                1,
-                1,
+                MULTI_BUFFER_EXCERPT_HEADER_HEIGHT,
+                MULTI_BUFFER_EXCERPT_FOOTER_HEIGHT,
                 fold_placeholder,
                 cx,
             )
@@ -1812,6 +1815,7 @@ impl Editor {
             git_blame_inline_enabled: ProjectSettings::get_global(cx).git.inline_blame_enabled(),
             blame: None,
             blame_subscription: None,
+            file_header_size,
             tasks: Default::default(),
             _subscriptions: vec![
                 cx.observe(&buffer, Self::on_buffer_changed),
@@ -10829,6 +10833,12 @@ impl Editor {
                 self.refresh_inlay_hints(InlayHintRefreshReason::ExcerptsRemoved(ids.clone()), cx);
                 cx.emit(EditorEvent::ExcerptsRemoved { ids: ids.clone() })
             }
+            multi_buffer::Event::ExcerptsEdited { ids } => {
+                cx.emit(EditorEvent::ExcerptsEdited { ids: ids.clone() })
+            }
+            multi_buffer::Event::ExcerptsExpanded { ids } => {
+                cx.emit(EditorEvent::ExcerptsExpanded { ids: ids.clone() })
+            }
             multi_buffer::Event::Reparsed => {
                 self.tasks_update_task = Some(self.refresh_runnables(cx));
 
@@ -11299,6 +11309,10 @@ impl Editor {
         }));
         self
     }
+
+    pub fn file_header_size(&self) -> u8 {
+        self.file_header_size
+    }
 }
 
 fn hunks_for_selections(
@@ -11743,6 +11757,12 @@ pub enum EditorEvent {
     ExcerptsRemoved {
         ids: Vec<ExcerptId>,
     },
+    ExcerptsEdited {
+        ids: Vec<ExcerptId>,
+    },
+    ExcerptsExpanded {
+        ids: Vec<ExcerptId>,
+    },
     BufferEdited,
     Edited,
     Reparsed,