[stub] d.ts should import files with '.js' extension in ESM package

[yshrsmz]

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🔗 Linked issue

resolve #370

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [x] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Updated DTS stub generation logic to check its module format so that it can work with ESM package.

I wanted to write test cases for this change, but there seems to be no stub-mode related tests and no idea how to write it 🤔

📝 Checklist

• [x] I have linked an issue or discussion.
• [ ] I have updated the documentation accordingly.