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

docs/src/visual-customization.md

@@ -368,6 +368,8 @@ TBD: Centered layout related settings
 
   // How to render LSP `textDocument/documentColor` colors in the editor.
   "lsp_document_colors": "inlay",        // none, inlay, border, background
+  // When to show the scrollbar in the completion menu.
+  "completion_menu_scrollbar": "never", // auto, system, always, never
 ```
 
 ### Edit Predictions {#editor-ai}