feat(core): spawn function

[aarthificial]

Makes it possible to run generators concurrently without having to yield them:

spawn(function*() {
  yield* rect().opacity(0, 1);
})

Or:

spawn(rect().opacity(0, 1))

Together with ref it makes it easy to run animations upon adding the node to the tree:

import {Circle, makeScene2D} from '@motion-canvas/2d';
import {waitFor} from '@motion-canvas/core';
import {spawn} from '@motion-canvas/core/lib/threading/spawn';

export default makeScene2D(function* (view) {
  view.add(
    <Circle
      ref={node => spawn(node.scale(1, 1))}
      scale={0}
      width={320}
      height={320}
      fill={'lightseagreen'}
    />,
  );

  yield* waitFor(5);
});

However we still need to improve the typing of ref to make it truly useful. Right now it's always typed as the base Node class.