function findMatchingParen(text, start) {
  let level = 0;
  for (let i=start, l=text.length; i < l; i++) {
    if (text[i] === '(') {
      level++;
    } else if (text[i] === ')') {
      if (--level === 0) {
        return i;
      }
    }
  }
  return -1;
}
Walk from text[start] matching parens and
returns position of the outer end paren
@param {string} text
@param {number} start
@return {number}