feat(rollup)!: default to esnext build target

[danielroe]

🔗 Linked issue

nuxt/module-builder#181 (comment)

❓ 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

Not quite sure how to categorise, but this bumps esbuild target from es2020 to esnext. However, we might consider using a year-bound target (https://esbuild.github.io/content-types/#javascript) to fix the feature-set.

📝 Checklist

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

[pi0]

This seems a fine default behavior to me to not modify used syntaxes unless configured to do as syntaxes like import assertion which not only are supported in Node.js/v8 but are also essential(!) do not get stripped.