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/ui/src/components/scrollbar.rs

@@ -392,7 +392,7 @@ pub struct Scrollbars<T: ScrollableHandle = ScrollHandle> {
 
 impl Scrollbars {
     pub fn new(show_along: ScrollAxes) -> Self {
-        Self::new_with_setting(show_along, |_| ShowScrollbar::Always)
+        Self::new_with_setting(show_along, |_| ShowScrollbar::default())
     }
 
     pub fn for_settings<S: ScrollbarVisibility>() -> Scrollbars {