Integrate with core's default content exporter

Problem/Motivation

Core is about to add a default content export system: #3532694: Add a command-line utility to export content in YAML format

It is extensible and modules can integrate with it by means of an event. ERR should integrate with this new system.

Proposed resolution

ERR should subscribe to the new PreExportEvent and set up a callback for ERR field items, ensuring that all properties are exported just as they would be by Default Content.