Improve two-fer: recognise re-assignment

**Is your feature request related to a problem? Please describe.**

When a solution is provided with a re-assignment, the analyzer bails out of a lot of code paths.

```javascript
export function twoFer(name = 'you') {
const whomst = name
return `One for me, and one for ${whomst}.`
}
```

When a solution is provided with a re-assignment of the named argument, the analyzer doesn't recognise the shadowing:

```javascript
export function twoFer(name) {
name = name ? name : "you"
return `One for me, and one for ${name}.`
}
```

**Which exercise**
`two-fer`

**Describe the solution you'd like**

Simple re-assignments like the one above should be recognised and _disapproved_. They don't add any value to the solution.

Re-assignments of the named arguments should be recognised and _disapproved_. https://github.com/exercism/javascript-analyzer/issues/52#issuecomment-750298061

まず、`two-fer` 演習の analyzer ロジックを追跡し、名前付き引数への代入をどのように処理しているかを確認します。Issue にある 2 つの例を比較し、次に単純な再代入のカバレッジを追加して、そのような解決策が却下されることを確認します。Payload にはソースファイルもテストパスも記載されていません。

索引モデルが issue の本文から書いたものです。