feat: unified CI jobs into one ci.yml

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🛠 Tooling: Unify CI GitHub Actions workflows into a single ci.yml #1099
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Comes with a nice overall reduction in line count too, despite the addition of createMultiWorkflowFile.ts.

💖

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (d502d3e) to head (2410b07).
Report is 109 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1608      +/-   ##
==========================================
- Coverage   98.09%   97.86%   -0.23%     
==========================================
  Files         102      172      +70     
  Lines        5764     9657    +3893     
  Branches      491      936     +445     
==========================================
+ Hits         5654     9451    +3797     
- Misses        109      205      +96     
  Partials        1        1              

Flag	Coverage Δ
create	70.79% <91.05%> (+0.07%)	⬆️
initialize	44.16% <12.10%> (-0.15%)	⬇️
migrate	70.75% <91.05%> (+0.08%)	⬆️
unit	90.46% <100.00%> (+11.95%)	⬆️

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[github-actions]

🎉 This is included in version v1.67.0 🎉

The release is available on:

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

Cheers! 📦🚀