Add exports field to package.json

[P4sca1]

https://nodejs.org/api/packages.html#conditional-exports

This should help with import errors in environments where module is not supported, but exports.import is.

[apollo-cla]

@P4sca1: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[P4sca1]

I signed the CLA a few months ago. Is there anything else I need to do?

[jpvajda]

@P4sca1 No, that's all you need to do to contribute! Thanks for making this PR, the team will review this soon.