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/editor/src/editor_settings.rs

@@ -55,6 +55,7 @@ pub struct EditorSettings {
     pub drag_and_drop_selection: DragAndDropSelection,
     pub lsp_document_colors: DocumentColorsRenderMode,
     pub minimum_contrast_for_highlights: f32,
+    pub completion_menu_scrollbar: ShowScrollbar,
 }
 #[derive(Debug, Clone)]
 pub struct Jupyter {
@@ -268,6 +269,7 @@ impl Settings for EditorSettings {
             },
             lsp_document_colors: editor.lsp_document_colors.unwrap(),
             minimum_contrast_for_highlights: editor.minimum_contrast_for_highlights.unwrap().0,
+            completion_menu_scrollbar: editor.completion_menu_scrollbar.map(Into::into).unwrap(),
         }
     }
 }