Function union

union<K1 extends Primitive, K2 extends Primitive>(
a: ISet<K1>,
b: ISet<K2>,
): ISet<K1 | K2>
Computes the union of two ISet instances.

Returns a new set containing all elements that are present in either input set. Duplicate elements are automatically handled since sets only contain unique values. This operation is commutative: union(a, b) === union(b, a).

Performance: O(n + m) where n and m are the sizes of the input sets.
Type Parameters
- K1 extends Primitive
  The type of elements in the first set.
- K2 extends Primitive
  The type of elements in the second set.
Parameters
- a: ISet<K1>
  The first set.
- b: ISet<K2>
  The second set.
Returns ISet<K1 | K2>
A new ISet instance containing all elements from both sets.
Example
```
const numbers = ISet.create([1, 2]);
const words = ISet.create(['one', 'two']);

const union = ISet.union(numbers, words);

assert.deepStrictEqual(Array.from(union), [1, 2, 'one', 'two']);
```
- Defined in src/collections/iset.mts:674