function buildPseudoNthVElementMatcher(testIndex) {
  return (element) => {
    const elements = element.parent.children.filter(isVElement)
    return testIndex(elements.indexOf(element), elements.length)
  }
}
Build VElementMatcher for :nth-xxx()
@param {(index: number, length: number)=>boolean} testIndex
@returns {VElementMatcher}