const/readonly assertion support for `Map` and `Set`

まだ誰も着手していません。

### 🔍 Search Terms

"as const", "as readonly", "const assertion", "keyed collections"

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

Have a way to take an existing `Map` or `Set` and assert it to be its readonly variant without needing to repeat the generic types.

### 📃 Motivating Example

Today we can create maps (and sets) with explicit and inferred generic types but in order to make them readonly the generic types have to be explicitly written, sometimes repeated:

```ts
const exampleMap1: ReadonlyMap = new Map([["a", 1], ["b", 2]]);
const exampleMap2: ReadonlyMap = new Map().set('a', 1).set('b', 2);
```

With something like const assertion support for `Map` (and `Set`) we'd be able to get readonly maps and sets without repeating the generic types:

```ts
const exampleMap1 = new Map([["a", 1], ["b", 2]]) as const;
const exampleMap2 = new Map().set("a", 1).set("b", 2) as const;
```

### 💻 Use Cases

1. What do you want to use this for? I want to use this for creating keyed collections that will be used elsewhere and I want to avoid modifications. (Note: this won't prevent runtime modifications but the same is true for const assertions on arrays and objects today.)
2. What shortcomings exist with current approaches? Repetative generic types and verbose code to make a keyed collection readonly.
3. What workarounds are you using in the meantime? Repeating the generic types and explicitly defining things as readonly keyed collections.

まず、配列とオブジェクトに対する TypeScript の const アサーションの動作と、既存の Map および Set の型チェック規則を確認します。ジェネリック型を繰り返さずに readonly のキー付きコレクションをどのように表現すべきかを判断し、提案するアサーションによって readonly の Map 型と Set 型が推論される一方で、現在の JavaScript 出力が維持されることを示すテストカバレッジを追加します。

索引モデルが issue の本文から書いたものです。