fix: add README.md contributors section in setup

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: All Contributors table not created in setup mode #1956
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

The previous pnpx all-contributors-cli command was only enough for transition mode to pick up on existing contributors and create a README.md section. For setup mode, there needed to be a default in options if .all-contributorsrc doesn't yet exist. That's now pulled from the GitHub API.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.46%. Comparing base (8ca464e) to head (2b88625).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1958      +/-   ##
==========================================
+ Coverage   95.43%   95.46%   +0.03%     
==========================================
  Files          87       89       +2     
  Lines        3591     3618      +27     
  Branches      207      210       +3     
==========================================
+ Hits         3427     3454      +27     
  Misses        164      164              

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