Add initializer property to JSDocParameterTag node

まだ誰も着手していません。

### 🔍 Search Terms

param, parameter, argument, jsdoc, initializer, default value, parameter

### ✅ 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

Add an optional initializer property to the `JSDocParameterTag`, which will contain the default value of the param tag.

For a function:

```js
/**
* @param {string} [p="test"] - An optional param with a default value
*/
const abc = (p) => {};
```

The `initializer` property will be `test`.

This is a parity change with the `Parameter` node, which has `initializer` property for initializer in the parameter declaration:

```ts
const abc = (p: string = "test") => {};
```

### 📃 Motivating Example

Adding the `initializer` property to the `JSDocParameterTag` makes it possible to indicate default value of any overloaded function or function declaration, which was previously impossible. This adds the same property which was previously only available on the `Parameter` node. So IDEs or other tools built on TypeScript compiler now can easily communicate the default value of the parameter.

Since the default value of the parameter is currently already parsed, it should by trivial to just add it to the node as a new property and shouldn’t cause any performance issues.

### 💻 Use Cases

1. What do you want to use this for?
This can be used by IDEs or other tooling (such as documentation generators) to show default value of the parameter, which rely on TypeScript compiler.
In my use case I’m building a documentation generator which outputs parameter table for functions with types, default values and descriptions. For default values I’m using the initializer of the function and if it’s not specified I would like the default value from the param tag.

2. What shortcomings exist with current approaches?

There is no way how to access default values of overloaded methods or method declarations. This is also true for overloaded methods where initializer is not permitted in the parameter itself.

3. What workarounds are you using in the meantime?

The tools which want to use this have to parse the jsdoc parameter themselves.

If this issue gets approved I would gladly submit a PR for this.

TypeScript コンパイラー内の JSDocParameterTag の定義と既存の Parameter initializer の処理を見つけ、JSDoc パラメーターのデフォルト値がどこで解析されるかを追跡します。JSDoc AST プロパティに関する関連するコンパイラー テストを確認し、解析されたデフォルト値が optional initializer として公開されることを示す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。