Array concat

Requirements

• Complete installMyConcat(), installing Array.prototype.myConcat. The scaffold installs it when imported. Keep native concat unchanged. Define the practice method as non-enumerable, writable, and configurable.
• For ordinary array receivers, array.myConcat(...items) returns a fresh array. Copy receiver elements first, followed by every argument in order. With no arguments, return a shallow copy.
• Arrays are spread one level by default. Other values, including strings and ordinary array-like records, are appended as single elements. Do not recursively flatten nested arrays or clone objects.
• Honor Symbol.isConcatSpreadable on the receiver and arguments: an undefined flag uses the array default, a truthy flag spreads indexed elements, and a falsy flag appends the whole value. Spreadable records have a numeric nonnegative integer length; assume total output length is at most 10,000.
• Preserve sparse slots and trailing holes. An existing indexed property with value undefined is not a hole. Copy inherited indexed properties too, using the in operator. Do not mutate any input.
• Do not use native concat, flat, or flatMap. Array subclasses/species, non-array receivers, proxies, arbitrary length coercion, and invalid spreadable lengths are outside this interview-sized contract.

Examples

Input: [1, 2].myConcat([3], 4)
Output: [1, 2, 3, 4]

Input: [1].myConcat([2, [3]])
Output: [1, 2, [3]]

Input: [1, , 3].myConcat([4])
Output: [1, <empty>, 3, 4]