fix: more permissive usage bounds and headings

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: options.usage reading removes existing contents in some repositories #2041
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Makes options.usage parsing more permissive in two ways:

• Instead of only searching for a second heading under ## Development, also allows ## Contributing and ## Contributors
• If that second heading isn't found then the rest of the README.md is returned instead of undefined

The fixed behavior can be seen in JoshuaKGoldberg/cspell-populate-words@736e48b.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.81%. Comparing base (5f58fb2) to head (bd9c686).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2042      +/-   ##
==========================================
- Coverage   99.81%   99.81%   -0.01%     
==========================================
  Files         118      118              
  Lines        4919     4916       -3     
  Branches      305      305              
==========================================
- Hits         4910     4907       -3     
  Misses          9        9              

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[github-actions]

🎉 This is included in version v2.15.6 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀