Deep Merge

Implement deepMerge(...objects), which merges any number of objects from left to right into a new object.

• Keys that exist in only one object are copied.
• When a key exists in several objects: if both values are plain objects, merge them recursively; otherwise the value from the later object wins.
• Arrays are not merged: an array from a later object replaces the earlier value.
• A key whose later value is undefined still overrides (it is a real value).
• The inputs are never modified, and nested plain objects in the result are fresh copies, so changing the result does not change an input.
• Own enumerable string keys only. The key "__proto__" must be ignored, so merging untrusted JSON cannot change Object.prototype.

A plain object is one created by {}, Object.create(null) or JSON.parse. Dates, class instances and other objects are treated as ordinary values (later wins, copied by reference).