code: function findOne(element, selector) {
    return (element = getElement(element)) ? _getElement(element, selector) : null;
  }
docs: Find the first element's child matching the selector.
  @param {(string|Array|NodeList|HTMLCollection|Element|Document|ParentNode)} element - The parent node. Note that it'll be passed to getElement to ensure there's only one.
  @param {string} selector - The selector to match.
  @return {(Element|null)} element - The first child of elements matching the selector or null.
  @example //esnext
  import { createElement, append, find } from 'chirashi'
  const maki = createElement('.maki')
  append(maki, ['.salmon[data-fish][data-inside]', '.avocado[data-inside]'])
  const roll = createElement('.roll')
  append(roll, '.tuna[data-fish][data-inside]')
  append(document.body, [maki, roll])
  findOne('div', '[data-fish]') //returns: <div class="salmon" data-fish data-inside></div>
  findOne(maki, '[data-inside]') //returns: <div class="salmon" data-fish data-inside></div>
  @example //es5
  var maki = Chirashi.createElement('.maki')
  Chirashi.append(maki, ['.salmon[data-fish][data-inside]', '.avocado[data-inside]'])
  var roll = Chirashi.createElement('.roll')
  Chirashi.append(roll, '.tuna[data-fish][data-inside]')
  Chirashi.append(document.body, [maki, roll])
  Chirashi.findOne('div', '[data-fish]') //returns: <div class="salmon" data-fish data-inside></div>
  Chirashi.findOne(maki, '[data-inside]') //returns: <div class="salmon" data-fish data-inside></div>