fix: splice badges in readme during migration

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: Existing description and badges not detected #751
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

• Detecting existing badges
• Splicing new badges alongside existing equivalents and other badges
• Inject into an existing <p>...</p> instead of prepending a new one

writeReadme's logic is complex enough now that I gave it its own writeReadme/index.ts.

Verified with unit tests in the PR as well as running JoshuaKGoldberg/all-contributors-auto-action#68 locally.

[codecov]

Codecov Report

Merging #847 (a6909ef) into main (858847a) will increase coverage by 0.22%.
Report is 6 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #847      +/-   ##
==========================================
+ Coverage   92.17%   92.40%   +0.22%     
==========================================
  Files          84       87       +3     
  Lines        4232     4345     +113     
  Branches      263      280      +17     
==========================================
+ Hits         3901     4015     +114     
+ Misses        331      330       -1     

Flag	Coverage Δ
create	71.44% <54.92%> (-0.43%)	⬇️
initialize	34.39% <13.14%> (-0.31%)	⬇️
migrate	71.41% <74.64%> (+0.15%)	⬆️
unit	53.88% <100.00%> (+1.32%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/create/createWithOptions.ts	69.11% <100.00%> (ø)
src/migrate/migrateWithOptions.ts	77.61% <100.00%> (ø)
src/steps/writeReadme/findExistingBadges.ts	100.00% <100.00%> (ø)
src/steps/writeReadme/findIntroSectionClose.ts	100.00% <100.00%> (ø)
src/steps/writeReadme/generateTopContent.ts	100.00% <100.00%> (ø)
src/steps/writeReadme/index.ts	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[github-actions]

🎉 This is included in version v1.29.48 🎉

The release is available on:

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

Cheers! 📦🚀