fix: prefer `nitro.static` over `_generate`

[danielroe]

🔗 Linked issue

❓ 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

Since nuxi v3.8, we've supported setting nuxt.options.nitro.static instead of nuxt.options._generate (which is an internal flag) - see https://github.com/nuxt/nuxt/pull/21860.

Now, in preparation for Nuxt v4, we've removed the types for _generate (see https://github.com/nuxt/nuxt/pull/32355). This PR adds support for new version in backwards compatible way (ignoring type issues) - I'd suggest you remove support in a future major.

📝 Checklist

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