[Enhancement] Add return type for vector functions

[supjs]

Is your feature request related to a problem? Please describe.
I am using PhpStorm and the automatically generated typings for the vector all use return type void.

Examples:

clone(): void;
add(v: Two.Vector): void;
sub(v: Two.Vector): void;

Describe the solution you'd like
Add a return type to the documentation so that it returns Two.Vector instead.

See functions like makeRectangle:

/**
 * @name Two#makeRectangle
 * @function
 * @param {Number} x
 * @param {Number} y
 * @param {Number} width
 * @param {Number} height
 * @returns {Two.Rectangle}
 * @description Creates a Two.js rectangle and adds it to the scene.
 */

[jonobr1]

Thanks for posting this and sorry for the delayed response. I'll track this on the es6 features branch here: https://github.com/jonobr1/two.js/tree/es6

[jonobr1]

I haven't tested this, but it looks like phpStorm can infer types from imported packages. Two.js's latest version has these included.