fix(types): bring types from less into namespace

[rchl]

🔗 Linked issue

❓ 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

The Less namespace is referenced in this file and the @types/less is one of the dependencies of @nuxt/types but less is not imported anywhere within the @nuxt/types package which causes the following error when running tsc in projects using @nuxt/types:

../../packages/types/config/build.d.ts:68:10 - error TS2503: Cannot find namespace 'Less'.

68   less?: Less.Options
            ~~~~

Projects can work around it by including less in types in tsconfig.json but they shouldn't need to.

📝 Checklist

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

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[rchl]

Note that import type Less from 'less' would trigger typescript hint about unused variable. That's why it's just an import for the sake of referencing it.