get_node() function should have generics in TypeScript

In C#, we are able to use the simplest and nice `` as a shortcut and cleaner version of `(T)` or `as` to convert the Node into other class that is inherited to Node:
```csharp
private Sprite sprite;

public override void _Ready()
{
sprite = GetNode("path/to/sprite");
}
```
But in typescript, we are not allowed to write `` or generics to `get_node()` like in C#. Instead, we use `as` to convert the Node into other class that is inherited to Node.
```typescript
private sprite: godot.Sprite;

public override _ready(): void {
this.sprite = this.get_node('path/to/sprite') as godot.Sprite;
}
```
I'd make a workaround by editing the godot.d.ts but it would be nice if it's built-in to the module.

godot.d.ts から始めて、issue に記載されている get_node() の宣言を見つけます。その宣言を更新して、呼び出し側が返される Node 型の TypeScript ジェネリックを指定できるようにし、その後、例で as cast が不要になっていることを確認します。

索引モデルが issue の本文から書いたものです。