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.

首先盘点仓库中的 .mjs 文件并检查 package.json，然后确定依赖扩展名的导入路径。当文件使用 .js、package.json 声明 "type": "module"，且所有受影响的导入都得到一致更新时，此更改即完成。