feat(core): add output.exports option

[MissGwen]

Summary

This PR adds an output.exports option to generate the exports field in package.json from build entries.

• Supports ESM and CJS outputs, including import and require conditions.
• Supports single and multiple entries.
• Skips generation in watch mode.

This is the first iteration and only covers exports. If the remaining fields proposed in the issue are still needed, I’d be happy to add main, module, and types in a follow-up PR.

Related links

• [Feature]: Add an option to automatically generate exports fields in package.json #1291

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[elecmonkey]

Thank you for your contribution! The team is still discussing and thinking through the implementation approach. We’ll share any updates or continue the discussion here once we make progress!

[MissGwen]

Thank you for your contribution! The team is still discussing and thinking through the implementation approach. We’ll share any updates or continue the discussion here once we make progress!

Thanks for the update! Once the approach is settled, I’d be happy to update this PR accordingly and help with the remaining work in #1291 if needed.