Batch Async Processing

Implement batchAsync(items, batchSize, asyncFn) that processes items in sequential batches.

Requirements

• Split items into batches of batchSize.
• Within each batch, run all items in parallel via asyncFn(item).
• Batches are sequential: batch 2 starts only after batch 1 completes.
• Return a flat array of all results in the original order.

Example

const results = await batchAsync(
  [1, 2, 3, 4, 5],
  2,
  async (x) => x * 10
);
// [10, 20, 30, 40, 50]