code: public static function getNestedList($column, $key = null, $seperator = ' ') {
      $instance = new static;
  
      $key = $key ?: $instance->getKeyName();
      $depthColumn = $instance->getDepthColumnName();
  
      $nodes = $instance->newNestedSetQuery()->get()->toArray();
  
      return array_combine(array_map(function($node) use($key) {
        return $node[$key];
      }, $nodes), array_map(function($node) use($seperator, $depthColumn, $column) {
        return str_repeat($seperator, $node[$depthColumn]) . $node[$column];
      }, $nodes));
    }
docs: Return an key-value array indicating the node's depth with $seperator
  
  @return Array