module: reintroduce package exports dot main

This reintroduces the dot main in exports as discussed in the previous
Node.js modules meeting.

The implementation includes both CommonJS and ES module resolution with
the associated documentation and resolver specification changes.

In addition to the dot main, "exports" as a string or direct fallback
array is supported as well.

Co-Authored-By: Geoffrey Booth <GeoffreyBooth@users.noreply.github.com>
PR-URL: https://github.com/nodejs/node/pull/29494


Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>