Implement a function unionBy(arrays, iteratee) that merges multiple arrays into one, keeping only unique values, determined by a transformation function or property name.

The function should:

• Accept multiple arrays and an iteratee.
• The iteratee can be either:
  • A function applied to each element to determine uniqueness.
  • A string representing a property key to check uniqueness.
• Preserve the order of the first appearance of elements.

Examples

unionBy([[2.1], [1.2, 2.3]], Math.floor); 
// [2.1, 1.2]

unionBy([[{ x: 1 }], [{ x: 2 }, { x: 1 }]], 'x'); 
// [{ x: 1 }, { x: 2 }]