Use language settings' prettier parsers as a fallback for files with no path (#12273)

Follow-up of
https://github.com/zed-industries/zed/pull/12095#issuecomment-2123230762
reverting back part of https://github.com/zed-industries/zed/pull/11558
that was related to `language.toml` parsing.

Now all extensions that define `prettier_parser_name` in their language
configs, will enable formatting untitled buffers without any extra
language settings like

```json
{
  "languages": {
    "JSON": {
      "prettier": {
        "allowed": true,
        "parser": "json"
      }
    }
  }
}
```



Release Notes:

- Improved ergonomics of untitled buffer formatting with prettier, no
extra language settings are needed by default.