fix: use correct extractCSS type

[g1eny0ung]

🔗 Linked issue

Close #784

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Add @type {boolean | typeof import('extract-css-chunks-webpack-plugin').PluginOptions} in JSDoc and then unbuild will generate the correct type.

📝 Checklist

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