Add exports field to all package.json files

[webmaster128]

See https://nodejs.org/api/packages.html#community-conditions-definitions

Part of #1758. Closes #1758

[webmaster128]

Turns out this is a breaking change for any use who imports some sort of file path instead of the top level module:

[…] preventing any other entry points besides those defined in "exports". This encapsulation allows module authors to clearly define the public interface for their package.

(https://docs.npmjs.com/cli/v11/configuring-npm/package-json#exports)

Nothing wrong with that but it deserves a warning in the CHANGELOG