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.

docs/src/configuring-zed.md

@@ -639,6 +639,12 @@ List of `string` values
 "snippet_sort_order": "bottom"
 ```
 
+4. Do not show snippets in the completion list at all:
+
+```json
+"snippet_sort_order": "none"
+```
+
 ## Editor Scrollbar
 
 - Description: Whether or not to show the editor scrollbar and various elements in it.