doc: explain esm options for package authors

* organize sections more hierarchically

* recommend always including "type" field, per 2019-06-19 meeting

Refs: https://github.com/nodejs/modules/issues/342#issuecomment-503733187

* expand discussion of publishing cjs/esm packages

PR-URL: https://github.com/nodejs/node/pull/29497


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>