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_ui/src/page_data.rs

@@ -6541,6 +6541,19 @@ fn language_settings_data() -> Vec<SettingsPageItem> {
             metadata: None,
             files: USER | PROJECT,
         }),
+        SettingsPageItem::SettingItem(SettingItem {
+            title: "Completion Menu Scrollbar",
+            description: "When to show the scrollbar in the completion menu.",
+            field: Box::new(SettingField {
+                json_path: Some("editor.completion_menu_scrollbar"),
+                pick: |settings_content| settings_content.editor.completion_menu_scrollbar.as_ref(),
+                write: |settings_content, value| {
+                    settings_content.editor.completion_menu_scrollbar = value;
+                },
+            }),
+            metadata: None,
+            files: USER,
+        }),
         SettingsPageItem::SectionHeader("Inlay Hints"),
         SettingsPageItem::SettingItem(SettingItem {
             title: "Enabled",