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/gpui/src/elements/uniform_list.rs

@@ -251,8 +251,6 @@ impl Element for UniformList {
         None
     }
 
-    // self.max_found_width = 0.0
-    //
     fn request_layout(
         &mut self,
         global_id: Option<&GlobalElementId>,