Implement mergeDeep(a, b)

Implement mergeDeep(a, b)

Write mergeDeep(a, b) that recursively merges b into a, without mutating either input, and returns a brand-new object:

• If both a[key] and b[key] are plain objects, merge them recursively.
• Otherwise, b[key] wins outright, this includes arrays: an array in b fully replaces one in a, it doesn't merge element-by-element.

mergeDeep({ a: { x: 1 }, tags: ["old"] }, { a: { y: 2 }, tags: ["new"] });
// { a: { x: 1, y: 2 }, tags: ["new"] }