feat: add declaration type in ESMExport

[tmg0]

🔗 Linked issue

Issue: #226

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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

Add optional field declarationType in ESMExport. If export type is declaration, this field should be a string value like interface type enum const or function.
This approach can solve the issue of not being able to distinguish what specific type 'declaration' is.
Issue: #226
Unimport issue: #318
Unimport pull request: fix re-export ts enum type

📝 Checklist

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