Improve fieldsForPreview to align with other fieldsFor methods

[LorenzoSapora]

Description:

In the abstract NovaResource we have fieldsForPreview(), but it doesn't do anything. Instead you have to add the ->showOnPreview() to existing fields in fields(). While this is passable, it lacks the flexibility afforded to us like fieldsForDetail() and the like.

Detailed steps to reproduce the issue on a fresh Nova installation:

• Create a resource
• Add

public function fieldsForPreview()
{
    return [
       ID::make('ID')
    ];
}

• View resource, no change