Clear and concise description of the problem

As as a developer using tsdown, I'd like to not expose the details of my package.json to external consumers.

Suggested solution

Add an exports.manifest option (which defaults to true for compatibility).

Alternative

Expose the package.json.

Additional context

Would also be nice to let us shave off the module & types field; yes, some tools still don't support exports, but we should be trying to push the ecosystem forward.

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.