--threads not working with `jlfmt`

https://github.com/domluna/JuliaFormatter.jl/blob/cccfe0b52e0e8d2df8aa149a3af1ea9b25737397/src/app.jl#L590

This line should read

```jl
        Threads.@threads for opts in collect(options_list)
```

because `@threads` can only distribute the workload if it knows the length of the list in advance, and `options_list` is, contrary to its name, a generator, with unknown length.