copied all files to a start and a finished folder in threejs-cube/

[joaojgabriel]

It might seem like a convoluted commit at first glance, but it's actually pretty simple.

The repository only had the finished version of the threejs-cube/ exercise, so I copied all its files into two folders, the only difference being that in the start/ folder the script.js (where the exercise happens) is empty. No other files have changes in neither of the folders.

Please let me know if there is a better practice for a commit like this one.