Implement flex layout

[xiaoiver]

If we want to place a paragraph in a container, keeping it centered horizontally and vertically:

const container = new Rect({
  x: 0,
  y: 0,
  width: 100,
  height: 100,
  display: 'flex',
  alignItems: 'center',
  justifyContent: 'center'
});
const paragraph = new Text({
  content: 'Line 1 \n Line 2 \n Line 3',
});

container.appendChild(paragraph);

So we need the help of layout engine such as Yoga.

Other flex layout implementations:

• https://github.com/pmndrs/react-three-flex/
• https://github.com/fireveined/pixi-flex-layout/
• Simplifying WebGL: Building an Effective Layout Engine
• troika-flex-layout
• taffy
• clay