Implement buildTree(flatNodes, idKey, parentKey)

Implement buildTree(flatNodes, idKey, parentKey)

Comment threads, org charts, category trees, all delivered flat from an API and rendered nested. Write buildTree(flatNodes, idKey, parentKey) that turns a flat array into a nested tree: each node gets a children array, and nodes whose parentKey is missing/null/unresolvable become top-level roots.

buildTree(
  [{ id: 1, parentId: null }, { id: 2, parentId: 1 }, { id: 3, parentId: 1 }],
  "id", "parentId"
);
// [{ id: 1, parentId: null, children: [
//    { id: 2, parentId: 1, children: [] },
//    { id: 3, parentId: 1, children: [] }
// ]}]