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

assets/icons/list_tree.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="lucide lucide-list-tree"><path d="M21 12h-8"/><path d="M21 6H8"/><path d="M21 18h-8"/><path d="M3 6v4c0 1.1.9 2 2 2h3"/><path d="M3 10v6c0 1.1.9 2 2 2h3"/></svg>

assets/keymaps/default-linux.json

@@ -439,6 +439,7 @@
       "ctrl-shift-p": "command_palette::Toggle",
       "ctrl-shift-m": "diagnostics::Deploy",
       "ctrl-shift-e": "project_panel::ToggleFocus",
+      "ctrl-shift-b": "outline_panel::ToggleFocus",
       "ctrl-?": "assistant::ToggleFocus",
       "ctrl-alt-s": "workspace::SaveAll",
       "ctrl-k m": "language_selector::Toggle",
@@ -562,6 +563,18 @@
       "ctrl-enter": "project_search::SearchInNew"
     }
   },
+  {
+    "context": "OutlinePanel",
+    "bindings": {
+      "left": "project_panel::CollapseSelectedEntry",
+      "right": "project_panel::ExpandSelectedEntry",
+      "ctrl-alt-c": "project_panel::CopyPath",
+      "alt-ctrl-shift-c": "project_panel::CopyRelativePath",
+      "alt-ctrl-r": "project_panel::RevealInFinder",
+      "shift-down": "menu::SelectNext",
+      "shift-up": "menu::SelectPrev"
+    }
+  },
   {
     "context": "ProjectPanel",
     "bindings": {
@@ -583,7 +596,10 @@
       "ctrl-backspace": ["project_panel::Delete", { "skip_prompt": false }],
       "ctrl-delete": ["project_panel::Delete", { "skip_prompt": false }],
       "alt-ctrl-r": "project_panel::RevealInFinder",
-      "alt-shift-f": "project_panel::NewSearchInDirectory"
+      "alt-shift-f": "project_panel::NewSearchInDirectory",
+      "shift-down": "menu::SelectNext",
+      "shift-up": "menu::SelectPrev",
+      "escape": "menu::Cancel"
     }
   },
   {

assets/keymaps/default-macos.json

@@ -475,6 +475,7 @@
       "cmd-shift-p": "command_palette::Toggle",
       "cmd-shift-m": "diagnostics::Deploy",
       "cmd-shift-e": "project_panel::ToggleFocus",
+      "cmd-shift-b": "outline_panel::ToggleFocus",
       "cmd-?": "assistant::ToggleFocus",
       "cmd-alt-s": "workspace::SaveAll",
       "cmd-k m": "language_selector::Toggle",
@@ -584,6 +585,18 @@
       "cmd-enter": "project_search::SearchInNew"
     }
   },
+  {
+    "context": "OutlinePanel",
+    "bindings": {
+      "left": "outline_panel::CollapseSelectedEntry",
+      "right": "outline_panel::ExpandSelectedEntry",
+      "cmd-alt-c": "outline_panel::CopyPath",
+      "alt-cmd-shift-c": "outline_panel::CopyRelativePath",
+      "alt-cmd-r": "outline_panel::RevealInFinder",
+      "shift-down": "menu::SelectNext",
+      "shift-up": "menu::SelectPrev"
+    }
+  },
   {
     "context": "ProjectPanel",
     "bindings": {

assets/settings/default.json

@@ -302,6 +302,29 @@
     /// when a directory has only one directory inside.
     "auto_fold_dirs": false
   },
+  "outline_panel": {
+    // Whether to show the outline panel button in the status bar
+    "button": true,
+    // Default width of the outline panel.
+    "default_width": 240,
+    // Where to dock the outline panel. Can be 'left' or 'right'.
+    "dock": "left",
+    // Whether to show file icons in the outline panel.
+    "file_icons": true,
+    // Whether to show folder icons or chevrons for directories in the outline panel.
+    "folder_icons": true,
+    // Whether to show the git status in the outline panel.
+    "git_status": true,
+    // Amount of indentation for nested items.
+    "indent_size": 20,
+    // Whether to reveal it in the outline panel automatically,
+    // when a corresponding outline entry becomes active.
+    // Gitignored entries are never auto revealed.
+    "auto_reveal_entries": true,
+    /// Whether to fold directories automatically
+    /// when a directory has only one directory inside.
+    "auto_fold_dirs": true
+  },
   "collaboration_panel": {
     // Whether to show the collaboration panel button in the status bar.
     "button": true,