Add scrollbars to outline panel (#19969)

Part of https://github.com/zed-industries/zed/issues/15324


![image](https://github.com/user-attachments/assets/4f32d585-9bd2-46be-8234-3658a71906ee)

Repeats the approach used in the project panel.

Release Notes:

- Added scrollbars to outline panel

---------

Co-authored-by: Nate Butler <nate@zed.dev>

crates/outline_panel/src/outline_panel_settings.rs

@@ -1,3 +1,4 @@
+use editor::ShowScrollbar;
 use gpui::Pixels;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
@@ -29,6 +30,23 @@ pub struct OutlinePanelSettings {
     pub indent_guides: IndentGuidesSettings,
     pub auto_reveal_entries: bool,
     pub auto_fold_dirs: bool,
+    pub scrollbar: ScrollbarSettings,
+}
+
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+pub struct ScrollbarSettings {
+    /// When to show the scrollbar in the project panel.
+    ///
+    /// Default: inherits editor scrollbar settings
+    pub show: Option<ShowScrollbar>,
+}
+
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+pub struct ScrollbarSettingsContent {
+    /// When to show the scrollbar in the project panel.
+    ///
+    /// Default: inherits editor scrollbar settings
+    pub show: Option<Option<ShowScrollbar>>,
 }
 
 #[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
@@ -85,6 +103,8 @@ pub struct OutlinePanelSettingsContent {
     pub auto_fold_dirs: Option<bool>,
     /// Settings related to indent guides in the outline panel.
     pub indent_guides: Option<IndentGuidesSettingsContent>,
+    /// Scrollbar-related settings
+    pub scrollbar: Option<ScrollbarSettingsContent>,
 }
 
 impl Settings for OutlinePanelSettings {