Replace naive movement system with Physics-based one

[philipjscott]

Since we're making a simple racing game, we'll need to handle collisions; it's inevitable that we'll need to use a physics library of some sort, or make our own. For now, we should swap out our naive movement algorithm (ie. incrementing positions depending on whether keys are pressed) for something complex.

I anticipate that we'll use https://github.com/liabru/matter-js for the physics engine. Handling collisions is outside the scope of this issue; we'll tackle that problem in another issue.

[philipjscott]

I'm going to be creating a repository for testing out the library on client-side; it'll be easier for us to derive a physics system that way.

[philipjscott]

http://www.asawicki.info/Mirror/Car%20Physics%20for%20Games/Car%20Physics%20for%20Games.html

[philipjscott]

http://www.iforce2d.net/b2dtut/top-down-car