Allow positioning media files

[zeezo887]

Description

I added a migration file to include a position column in the mediable and fileable tables, enabling the retrieval of items in a specific order based on their position.

Additionally, there is a bug in the $this->attachNew($records, $current, false) method when updating existing pivot records. The issue arises because:

$this->newPivotStatementForId($this->parseId($id))->update(
    $this->castAttributes($attributes)
);

public function newPivotStatementForId($id)
{
    return $this->newPivotQuery()->whereIn($this->relatedPivotKey, $this->parseIds($id));
}

This code uses the relatedPivotKey (such as media_id or file_id) to locate the record for updating. However, in our case, the IDs passed to this method are the IDs of the pivot table itself, not media_id or file_id. To resolve this, I overrode the attachNew method and filtered by the pivot table ID:

$updated = $this->newPivotQuery()->whereIn('id', $this->parseIds($id))->update(
    $this->castAttributes($attributes)
);

Related Issues

Fixes #2690 , #2689