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. Pathauto should integrate with this new system.

Proposed resolution

It is possible that nothing will actually be necessary; core naïvely exports the Path field items, only removing the non-portable path ID.

But if it turns out that core is not exporting the Pathauto flag, then Pathauto should subscribe to the new PreExportEvent and wrap around the core callback for path field items, ensuring that the flag is preserved on export.