feat: provide static process.* and import.meta.* build flags

[pi0]

🔗 Linked issue

❓ 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

Provide static flags. These are replaced at build time and can be used to tweak build output bundle based on them. (Therefore are typed as optional since externals won't have them)

• process.dev / import.meta.dev: true for development mode otherwise false
• process.client / import.meta.client: Always false
• process.server / import.meta.server: Always true
• process.nitro / import.meta.nitro: Always true
• process.prerender / import.meta.prerender: When in prerender mode
• process.preset / import.meta.preset: Nitro build preset
• process.versions.nitro / import.meta.versions.nitro

📝 Checklist

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