🛠️ Repo: Convert `lib/utils.js` to ESM

[JoshuaKGoldberg]

Sub-issue of #5400 (Switch from CJS to ESM internally).

lib/utils.js is a widely-consumed utility module still written in CJS. It exports many helper functions via exports.X and is a dependency across much of the core runtime.

File to convert

• lib/utils.js

Notes

Per AGENTS.md CJS-to-ESM conventions:

• Prefer named exports over export default
• Rename to utils.mjs
• Update all consumers to use const { ... } = require("./utils.mjs") (destructured named imports)
• There are many call sites across lib/ — a search for require.*utils will surface them all