types: Add generic types to Container-like views

[CatchABus]

Please Describe The Problem To Be Solved
It would be a nice addition if DisplayObjects were more type-friendly when it comes to handling children.
Pixi itself does that with Container.

const container = new Container<Sprite>();
// Now I can manipulate children without casting type

(Optional): Suggest A Solution
Example using a List

const list = new List<Graphics>({
    children: [
        new Graphics().beginFill(0x000000).drawRect(0, 0, 50, 50),
        new Graphics().beginFill(0xFFFFFF).drawRect(0, 0, 50, 50),
    ],
 });

console.log(list.children); // Type of Graphics[]

[CyberDex]

You are welcome to create this PR ;)

[CatchABus]

Closed via #191