Parses patterns like \1, \2, etc. either as a backreference
to a group, or a deciaml char code.
function GroupRefOrDecChar(text, textLoc) {
  const reference = Number(text.slice(1));

  if (reference > 0 && reference <= capturingGroupsCount) {
    return Node({
      type: 'Backreference',
      kind: 'number',
      number: reference,
      reference,
    }, textLoc);
  }

  return Char(text, 'decimal', textLoc);
}