Add `exports` to package DSL

[iruizmar]

NpmPackage is missing a property to typesafe set the exports option of an NPM package as defined here:
https://nodejs.org/api/packages.html#subpath-exports

Please, keep in mind that inside each exported package, many properties may be set, for example, Typescript will search for a "types" property inside each module as stated here:

https://www.typescriptlang.org/docs/handbook/modules/reference.html#packagejson-exports

Thanks!