feat: sourcemap support

[pi0]

🔗 Linked issue

Resolves #236 also #47, #234

❓ 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

This feature adds (more experimental) sourcemap support via top level sourcemap: true option and CLI --sourcemap (disabled by default).

Behavior can be fine-tuned for rollup using rollup.output.sourcemap and for esbuild using rollup.esbuild.sourcemap.

Unbuild support requires unjs/mkdist#164

📝 Checklist

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