Related issue #48600, following #48904

Setting `"formatter": null` does not actually disable the formatter, as
it is functionally equivalent to leaving the setting undefined. In this
case, Zed falls back to the default behavior—either "auto" or a specific
language server, depending on the language.

This PR adds a "none" option to the formatter setting, allowing users to
explicitly disable formatting while keeping `code_actions_on_format`
enabled separately.
```json
"formatter": "none"
```

I still have some doubts about the necessity of this setting itself, but
if it is a desired addition, I will update the documentation
accordingly. If not, please feel free to close this PR.

- [x] Tests or screenshots needed?
- [x] Code Reviewed
- [x] Manual QA

Release Notes:

- Added `"formatter": "none"` in settings to explicitly disable the
formatter.