Support `@deprecated` marker on namespace aliases

まだ誰も着手していません。

### 🔍 Search Terms

"@deprecated", "deprecated", "namespace", "namespace alias"

### ✅ 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

Note: Marking this as a feature request, very well might be a bug. It's one of those that are probably up for interpretation?

---

I'd like the ability to annotate namespace aliases with `@deprecated`. At the moment you'd need to mark the referenced type as deprecated, which often isn't viable.

Here's a little example:

```ts
// @filename: example.d.ts
export type Bar = string | number;

/** @deprecated */
export type Baz = string | number;

// @filename: foo.ts
import * as example from './example';

declare namespace JSX {
/** @deprecated */
export type Foo = string | number;

/** @deprecated */
export import Bar = example.Bar;

export import Baz = example.Baz;
}

let x: JSX.Foo; // Marked as deprecated
let y: JSX.Bar; // Not marked as deprecated (but should be)
let z: JSX.Baz; // Marked as deprecated
```

[TypeScript Playground Link](https://www.typescriptlang.org/play/?jsx=0&multiFile=true#code/PTAEAEDMEsBsFMB2BDAtvAXKeAPNAHBAOgBMiAXAZwChd8B7AJ3NHIE995QAhZR0ALyhK5RtEQBzUAB9QiAK6oARvEYBuatWAAqbRBLx8jeAGNk5eCVDbgtHA2asOXXgC9Bw0eKmyFy1RqaIBAwCCjoWJD09BQ00KgOLHrIlNh4CQigkIz0qKAA5ETAuAQI+YEGJrB8XOHwlPjIJlwAUgDKABqgAN7UAJA6euAGRqbmlta2fXRMLOycoABi0R4iYpIycooq6poDuvqGxmYWVjb9M47xiTx8HiUZ8ES8uxf2s6DXH2736YRPbg0AF9NAgWDgsO0OkRlvQNGDQGxIZ1nnx4fAWK5kdDAUA)

### 📃 Motivating Example

Allows developers to mark namespace aliases as deprecated, giving downstream users more warning when those aliases will go away.

### 💻 Use Cases

1. What do you want to use this for?

[preact](https://github.com/preactjs/preact) has long since loaded up the JSX namespace with unrelated types... shouldn't have, but did. We're looking to get rid of this in our next major and to help folks migrate we wanted to extract the types to the `preact` namespace and re-export them as aliases through the `JSX` namespace. This will avoid any breaking changes in the current version and allow folks to keep compatibility with v10 and the upcoming v11.

Unfortunately, however, we can't mark this usage as deprecated which is a shame.

2. What shortcomings exist with current approaches?

Just doesn't work. End users get no warning that the namespace alias is deprecated.

3. What workarounds are you using in the meantime?

Just duplicated all the types. Instead of extracted types that are consumed both as direct exports and namespace aliases, we've had to make copies. Those in the namespace get the deprecation comment, those outside do not.

It's pretty fragile obviously, any change to one now has to be reflected to the same interface in another file.

Issue の TypeScript Playground の例から始め、namespace メンバーと型エイリアスについて非推奨がどのように報告されるかを追跡します。`export import Bar = example.Bar` の回帰テストを追加し、`JSX.Bar` が非推奨としてマークされる一方で、既存の `JSX.Baz` の動作は変更されないようにします。namespace エイリアスが期待される非推奨警告を生成し、JavaScript の出力を変更しなければ完了です。

索引モデルが issue の本文から書いたものです。