Add created and changed entity properties to markdown front matter

Problem/Motivation

When exporting content to markdown, the front matter entity block did not include the created and changed timestamps. These are standard entity properties that tools (including LLMs) rely on to understand when content was authored or last updated.

The initial entity_properties config introduced in content_first_update_20001() only covered id, type, bundle, status, and langcode.

Proposed resolution

Add created and changed as configurable entity properties:

• Expose them as checkboxes in the Entity properties section of the Content First settings form.
• In ContentFirstBuilder, read the created value via $entity->get('created')->value and the changed value via $entity->getChangedTime() (using EntityChangedInterface).
• Add a post-update function content_first_update_20002() to enable both properties for existing installations.

Remaining tasks

• Review and merge.

User interface changes

The Entity properties details element in the Content First settings form gains two new checkboxes: Created date (created) and Changed date (changed). Both are enabled by default