Consider porting .mjs files to .js with "type": "module"

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.

まずリポジトリの .mjs ファイルを一覧化して package.json を確認し、その後、拡張子に依存する import パスを特定します。ファイルが .js を使用し、package.json が "type": "module" を宣言し、影響を受けるすべての import が一貫して更新されれば、変更は完了です。