feat(openapi): add experimental /_nitro/scalar endpoint

[hanspagel]

🔗 Linked issue

n/a

❓ 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

Nitro looks so cool! I saw you've added Swagger UI. We’re building a more modern OSS alternative to Swagger UI here: https://github.com/scalar/scalar It’s already integrated by default, or available as a package in/for Express, Fastify, Platformatic, Hono, ElysiaJS, Next.js, Laravel … and more. 😇

This PR adds /_nitro/scalar as an additional route to render OpenAPI files. And we’ve added a custom theme that matches the Nitro docs. What do you think? Really looking forward to your feedback!

nitrovideo.mp4

📝 Checklist

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

[pi0]

This is amazing! Let's merge and iterate over this.

I think eventually we might use scalar as nitro's default OpenAPI explorer!