Consider porting .mjs files to .js with "type": "module" #1850
Description
The v6 migration converted all files from CommonJS to ESM using the .mjs extension. The .mjs extension was a transitional mechanism — the intended long-term approach is "type": "module" in package.json, which makes plain .js files ES modules by default.
This is now the ecosystem convention (Node.js docs, major frameworks, etc.). Keeping .mjs signals an incomplete migration.
The change is mechanical — rename all .mjs → .js, add "type": "module" to package.json, update any import paths — but worth doing for consistency with modern Node.js conventions.