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

assets/settings/default.json

@@ -255,6 +255,19 @@
   // Whether to display inline and alongside documentation for items in the
   // completions menu
   "show_completion_documentation": true,
+  // When to show the scrollbar in the completion menu.
+  // This setting can take four values:
+  //
+  // 1. Show the scrollbar if there's important information or
+  //    follow the system's configured behavior
+  //   "auto"
+  // 2. Match the system's configured behavior:
+  //    "system"
+  // 3. Always show the scrollbar:
+  //    "always"
+  // 4. Never show the scrollbar:
+  //    "never" (default)
+  "completion_menu_scrollbar": "never",
   // Show method signatures in the editor, when inside parentheses.
   "auto_signature_help": false,
   // Whether to show the signature help after completion or a bracket pair inserted.