function unwrapParen (_node, parenthesis) {
  if (!parenthesis || parenthesis !== 'auto') return _node
  let node = _node
  while (isParenthesisNode(node)) node = node.content
  return node
}
Returns the first non-parenthesis internal node, but only
when the 'parenthesis' option is unset or auto.
@param {Node} _node
@param {string} parenthesis
@return {Node}