Mathematical operations for Vector2 signals

[aarthificial]

Description
Modifying a vector by applying some operation (addition, subtraction, etc) is currently really verbose.
Imagine you want to animate the position of node by moving it by [200, 400] relative to its current position:

yield* node.position(node.position().add([200, 400]), 0.6);

In general, whenever we want to derive the new value of a signal using its current value things become verbose.

Proposed solution
Introduce a set of methods for Vector2 signals that reflect the operations of the Vector2 class.

// before
yield* node.position(node.position().add([200, 400]), 0.6);
// after
yield* node.position.add([200, 400], 0.6);

These could be used to either tween or immediately modify the signal, just like you normally would:

// set
node.position.add([200, 400]);
// tween
yield* node.position.add([200, 400], 0.6);

On top of that, a new edit method should be introduced to derive the new value in whatever way necessary:

// this:
node.position.add([200, 400]);
// is the same as this:
node.position.edit(current => current.add([200, 400]));

While on itself still pretty verbose, this interface would allow you to create reusable functions for operations that are not natively supported.
Imagine you want to snap the vector to a 20x20 grid:

// define a reusable method:
function snap(grid: PossibleVector2) {
  return (current: Vector2) => current.div(grid).floored.mul(grid);
}

// use it:
node.position.edit(snap(20));

[mancopp]

I am working on this one