title: How to map json item value to sibling array with jq
body: <p>Having json like</p>
  
  <pre><code>[
    {
      "parent": "x",
      "children": ["a", "b"]
    },
    {
      "parent": "y",
      "children": ["c", "d", "e"]
    }
  ]
  </code></pre>
  
  <p>how to transform it with jq to the array of "[parent, child_order_number, child]" items like</p>
  
  <pre><code>[
    ["x", 0, "a"],
    ["x", 1, "b"],
    ["y", 0, "c"],
    ["y", 1, "d"],
    ["y", 2, "e"]
  ]
  </code></pre>
  
  <p>?</p>