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.

Start in godot.d.ts and locate the get_node() declaration described in the issue. Update that declaration so callers can provide a TypeScript generic for the returned Node type, then verify that the example no longer needs an as cast.

Written by the indexing model from the issue text.