Add utility to get/set ex.Vector size

Context

It would be useful to set the magnitude of a vector, this allows easy scaling or capping at a maximum value. This is especially useful in physics simulations.

Proposal

I propose something like the following (there is probably a more efficient method of changing the size of the vector)

  /**
   * The magnitude (size) of the Vector
   * @obsolete magnitude will be removed in favor `.size` in version v0.24.0
   */
  public magnitude(): number {
    return this.distance();
  }
  
 /**
  * The size (aka magnitude) of the Vector
  */
  public get size(): number {
    return this.distance();
  }

  public set size(newLength: number) {
    this.normalize().scale(newLength);
  }

Usage:

const vel = new ex.Vector(3, 4);

console.log(vel.size); // 5
vel.size = 13;

// Automatically calculates to correct scale
console.log(vel.x); // 5
console.log(vel.y); // 12

[eonarheim]

@saahilk Let me know if this doesn’t cover everything https://github.com/excaliburjs/Excalibur#environment-setup, please let me know if there are any gaps in the readme 👍

[saahilk]

@eonarheim Could you check the modified pull request here.