Update NodeTypeInterface::get/setPreviewMode to use NodePreview enum

Problem/Motivation

Follow-up to 📌 Each class using DRUPAL_* constants from system.module should define its own constants Needs work

We will have an enum for NodePreview states which can be used in NodeTypeInterface::get/setPreviewMode

Proposed resolution

Not sure the best way to do this while maintaining BC, most likely:

- Add deprecated $returnAsInt param to getPreviewMode and update return type to int|NodePreview
- Update usages in core of getPreviewMode to expect NodePreview enum
- Allow int|NodePreview in setPreviewMode
- Update usages in core of setPreviewMode to use the NodePreview enum

Remaining tasks

Do it

API changes

NodeTypeInterface::getPreviewMode and NodeTypeInterface::setPreviewMode now interact with the NodePreview enum

Release notes snippet