Add a way to output the `--dry-run` flag as a JSON

### Describe the feature or problem you’d like to solve

It would be great if I could run something like
```
gh pr create --dry-run --json
```
and get the output as a json.

This feature would help a lot when creating libraries that wrap the gh cli project.

### Proposed solution

I've yet to look at the source code, but I guess that the json creation part has already been done (since I suppose that github uses JSON for its API)