--json output should include option to surface the reviewer(s) of a PR

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

Currently `gh search prs` appears to allow filtering by PR reviewer with`--reviewed-by `.

The `--json` output enables outputting `author` and `assignees` but does not surface the reviewer(s) of a PR.

### Proposed solution

Add a `reviewers` option to the `--json` output