🛠️ Repo: Convert `lib/reporters/base.js` to ESM

[JoshuaKGoldberg]

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

lib/reporters/base.js is the last remaining CJS file among reporters. All other reporters (doc, dot, html, json, json-stream, landing, list, markdown, min, nyan, progress, spec, tap, xunit) have already been converted to .mjs.

File to convert

• lib/reporters/base.js

Notes

Per AGENTS.md CJS-to-ESM conventions:

• Prefer named exports over export default
• Rename to base.mjs
• Update all consumers to use const { ... } = require("./base.mjs") (destructured named imports)
• The lib/reporters/index.js CJS re-export shim will need to be updated to reference base.mjs