Fix `use_on_type_format` setting being unused per language (#18387)

Before this change, `use_on_type_format` would only have an effect when
defined on a global level in our settings.

But our default.json settings would also document that it's used in
language settings, i.e.:

```json
{
  "languages": {
    "C": {
      "use_on_type_format": false
    },
    "C++": {
      "use_on_type_format": false
    }
  }
}
```

But this did **not** work.

With the change, it now works globally and per-language.

Release Notes:

- Fixed `use_on_type_format` setting not working when defined inside
`"languages"` in the settings. This change will now change the default
behavior for C, C++, and Markdown, by turning language server's
`OnTypeFormatting` completions off by default.

Co-authored-by: Bennet <bennet@zed.dev>