feat: add packageManager back to package.json, and set to pnpm 10.4.0 in CI

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Add packageManager back to the root package.json #1906
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds a pnpm option to the Base, which defaults to the existing packageManager's pnpm version if it exists.

🎁

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.43%. Comparing base (1320f29) to head (4a9f6b7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/base.ts	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1961      +/-   ##
==========================================
- Coverage   95.47%   95.43%   -0.04%     
==========================================
  Files          89       89              
  Lines        3622     3635      +13     
  Branches      210      213       +3     
==========================================
+ Hits         3458     3469      +11     
- Misses        164      166       +2     

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[JoshuaKGoldberg]

Test timeout. Amusing.

[github-actions]

🎉 This is included in version v2.2.0 🎉

The release is available on:

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

Cheers! 📦🚀