Content entity reference props are not fully stable yet, so the code editor should only show this prop type when an explicit dev feature flag is enabled. Add a hidden `canvas_dev_er` module. When enabled, Canvas exposes `drupalSettings.canvas.devEntityReferenceMode`. The code editor should show the Content entity reference prop type only when that setting is `true`. Existing components that already use content entity reference props should continue to show that prop type so they remain editable.