build: update package.json entries with explicit mjs / mts extensions

[michaelfaith]

PR Checklist

• Addresses an existing open issue: fixes #000
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

(This was failing in the renovate PR: https://github.com/JoshuaKGoldberg/package-json-validator/pulls)

Following rolldown/tsdown#517, fixedExtension is now on by default when the platform is node, which means that it's emitting mjs for any esm js files and .d.mts for esm ts files.

[github-actions]

👋 Hi @michaelfaith, thanks for the pull request! A scan flagged some concerns with it. Could you please take a look?

[pr-task-completion] This PR's body is missing [x] checks on the following tasks from the PR template.

• Addresses an existing open issue: fixes #000
• That issue was marked as status: accepting prs

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[pr-linked-issue] This pull request is not linked as closing any issues. This repository keeps to GitHub issues for discussing potential changes. Most or all changes should be marked as approved in an issue before a pull request is sent to resolve them. To resolve this report:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.17%. Comparing base (91a08e7) to head (f32c91a).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #565   +/-   ##
=======================================
  Coverage   94.17%   94.17%           
=======================================
  Files          34       34           
  Lines         756      756           
  Branches      240      240           
=======================================
  Hits          712      712           
  Misses         34       34           
  Partials       10       10           

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[github-actions]

🎉 This is included in version v0.59.0 🎉

The release is available on:

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

Cheers! 📦🚀