Add directionTo() to Vector2 and Vector3

docs/api/vector2.md

 
 The length of the vector.
 
+### directionTo
+
+`vector.directionTo(v: Vector2): Vector2`
+
+Compute a unit vector pointing to `v` from `this`.
+
+**Arguments**
+
+* `v: Vector2`. A vector.
+
+**Returns**
+
+A new unit vector pointing to `v` from `this`.
+
 ### equals
 
 `vector.equals(v: Vector2): boolean`

docs/api/vector3.md

 
 The length of the vector.
 
+### directionTo
+
+`vector.directionTo(v: Vector3): Vector3`
+
+Compute a unit vector pointing to `v` from `this`.
+
+**Arguments**
+
+* `v: Vector3`. A vector.
+
+**Returns**
+
+A new unit vector pointing to `v` from `this`.
+
 ### equals
 
 `vector.equals(v: Vector3): boolean`

src/geometry/vector2.ts

     }
 
     /**
+     * Compute the direction from this to v
+     * @param v a vector
+     * @returns a new unit vector pointing to v from this
+     */
+    directionTo(v: Vector2): Vector2
+    {
+        return v._clone()._subtract(this)._normalize();
+    }
+
+    /**
      * Check if this and v have the same coordinates
      * @param v a vector
      * @returns true if this and v have the same coordinates
     }
 
     /**
+     * Clone this vector
+     * @returns a clone of this vector
+     * @internal
+     */
+    _clone(): Vector2
+    {
+        return new Vector2(this.x, this.y);
+    }
+
+    /**
      * Set the coordinates of this vector
      * @param x x-coordinate
      * @param y y-coordinate
     }
 
     /**
-     * Clone this vector
-     * @returns a clone of this vector
-     * @internal
-     */
-    _clone(): Vector2
-    {
-        return new Vector2(this.x, this.y);
-    }
-
-    /**
      * Normalize this vector
      * @returns this vector, normalized
      * @internal

src/geometry/vector3.ts

     }
 
     /**
+     * Compute the direction from this to v
+     * @param v a vector
+     * @returns a new unit vector pointing to v from this
+     */
+    directionTo(v: Vector3): Vector3
+    {
+        return v._clone()._subtract(this)._normalize();
+    }
+
+    /**
      * Check if this and v have the same coordinates
      * @param v a vector
      * @returns true if this and v have the same coordinates
     }
 
     /**
+     * Clone this vector
+     * @returns a clone of this vector
+     * @internal
+     */
+    _clone(): Vector3
+    {
+        return new Vector3(this.x, this.y, this.z);
+    }
+
+    /**
      * Set the coordinates of this vector
      * @param x x-coordinate
      * @param y y-coordinate
     }
 
     /**
-     * Clone this vector
-     * @returns a clone of this vector
-     * @internal
-     */
-    _clone(): Vector3
-    {
-        return new Vector3(this.x, this.y, this.z);
-    }
-
-    /**
      * Normalize this vector
      * @returns this vector, normalized
      * @internal