String.split() method's separator argument should allow undefined

### ⚙ Compilation target

ES2024

### ⚙ Library

6.0.2

### Missing / Incorrect Definition

String.split should allow undefined as its separator argument, resulting in an array containing the entire string.

Unlike the case of omitting the separator (#38331), this is a useful feature, since separator may be a variable, and it can be desirable to get an array with the entire string in the case that separator is not set, in order to not need a different code path for this case.

This feature is explicitly defined in the spec. It is not something that just 'happens to work', like using null or a number.

### Sample Code

```TypeScript
console.log("string,that could/split:but-should;not".split(undefined));
```

### Documentation Link

https://tc39.es/ecma262/multipage/text-processing.html#sec-string.prototype.split

リンク先のECMAScript仕様に照らしてString.splitの宣言を確認し、separator引数と提示されたサンプルに焦点を当てます。TypeScript定義がundefinedを受け入れ、別のコードパスを必要とせずにサンプルの型チェックが通れば完了です。

索引モデルが issue の本文から書いたものです。