Allow disabling snippet completion by setting snippet_sort_order to none (#34565)

This mirrors VSCode setting that inspired `snippet_sort_order` to begin
with; VSCode supports inline/top/bottom/none, with none completely
disabling snippet completion. See
https://code.visualstudio.com/docs/editing/intellisense#_snippets-in-suggestions

This is helpful for LSPs that do not allow configuring snippets via
configuration such as clangd.

Release Notes:

- Added `none` as one of the values for `snippet_sort_order` to
completely disable snippet completion.

assets/settings/default.json

@@ -197,6 +197,8 @@
   //    "inline"
   // 3. Place snippets at the bottom of the completion list:
   //    "bottom"
+  // 4. Do not show snippets in the completion list:
+  //    "none"
   "snippet_sort_order": "inline",
   // How to highlight the current line in the editor.
   //