editor: Add a setting to show a scrollbar in completion menu (#41849)

A user on Discord requested this feature:
https://discord.com/channels/869392257814519848/1434188637389717556/1434188637389717556

I added a scrollbar setting called `completion_menu_scrollbar` to the
completion menu and defaulted it to "Never" to match past behavior.

Release Notes:

- editor: Add `editor.completion_menu_scrollbar` setting to show a
scrollbar in the completion menu

crates/settings/src/settings_content/editor.rs

@@ -197,6 +197,19 @@ pub struct EditorSettingsContent {
     ///
     /// Default: [`DocumentColorsRenderMode::Inlay`]
     pub lsp_document_colors: Option<DocumentColorsRenderMode>,
+    /// When to show the scrollbar in the completion menu.
+    /// This setting can take four values:
+    ///
+    /// 1. Show the scrollbar if there's important information or
+    ///    follow the system's configured behavior
+    ///   "auto"
+    /// 2. Match the system's configured behavior:
+    ///    "system"
+    /// 3. Always show the scrollbar:
+    ///    "always"
+    /// 4. Never show the scrollbar:
+    ///    "never" (default)
+    pub completion_menu_scrollbar: Option<ShowScrollbar>,
 }
 
 // Toolbar related settings

crates/settings/src/vscode_import.rs

@@ -299,6 +299,7 @@ impl VsCodeSettings {
             toolbar: None,
             use_smartcase_search: self.read_bool("search.smartCase"),
             vertical_scroll_margin: self.read_f32("editor.cursorSurroundingLines"),
+            completion_menu_scrollbar: None,
         }
     }