[mark-wiemer]

PR Checklist

• Addresses an existing open issue: fixes 🛠️ Repo: Switch from CJS to ESM internally #5400
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

My version of #6034, rebased to reduce merge conflicts and respect Josh's fork

• Changes package.json to "type": "module" so Mocha is now an ESM-first package (users can still require('mocha'))
• Renames js files to cjs and mjs files to js
• Revert options.mjs to CJS (options.cjs) and some related files back to CJS due to rewiremock usage and low ROI for implementing (risky!) replacement. Turns out options.spec.mjs wasn't running in main (glob pattern) and when we made it run as ESM, it failed miserably!
• Fixes an internal bug with running nested tests in the unit folder, adding 1 test file (all tests pass)

[codecov]

Codecov Report

❌ Patch coverage is 97.27273% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.25%. Comparing base (7c9b004) to head (774c48b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.wallaby.cjs	0.00%	1 Missing ⚠️
bin/mocha.js	0.00%	1 Missing ⚠️
lib/cli/index.cjs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6078      +/-   ##
==========================================
+ Coverage   78.83%   82.25%   +3.41%     
==========================================
  Files          63       61       -2     
  Lines        4546     4564      +18     
  Branches      989      958      -31     
==========================================
+ Hits         3584     3754     +170     
+ Misses        962      810     -152     

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[JoshuaKGoldberg]