fix: resolve mjs extension before js

[chenjiahan]

Summary

Resolve .mjs extension before .js, to align with common practices.

• Node.js / TypeScript
  • does not support omitting a .mjs file extension
  • https://www.typescriptlang.org/docs/handbook/modules/reference.html#extensionless-relative-paths
• Bun
  • .mjs > .js
  • ['.tsx', '.jsx', '.ts', 'mjs', '.js', '.cjs', '.json']
  • https://bun.sh/docs/runtime/modules
• Vite
  • .mjs > .js
  • ['.mjs', '.js', '.mts', '.ts', '.jsx', '.tsx', '.json']
  • https://vitejs.dev/config/shared-options.html#resolve-extensions
• webpack / Rspack
  • does not support .mjs by default
  • ['.js', '.json', '.wasm']
  • https://webpack.js.org/configuration/resolve/#resolveextensions
  • https://rspack.dev/config/resolve#resolveextensions

Related Links

rspack-contrib/storybook-rsbuild#76

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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