Updated to ES6 and bundled with Rollup

[robertcorponoi]

Sorry for the huge PR but this couldn't be an incremental change.

• Added src folder that contains the new individual classes for Box, Circle, Polygon, etc.
• Kept the same code but changed vars to lets/consts where applicable.
• Added @babel/core and @babel/present-env packages and .babelrc file so that the new ES6 code can transpiled to ES5.
• Added rollup.config.js file so that a umd build can be created with npm run bundle or npm run bundle:watch.
• Created new SAT.js file using Rollup and ran the tests to make sure they all still passed.

[jriecken]

Sorry, I've been super busy and haven't had time to look at this. I'm not 100% sure when I can get to it, but be assured I'm not ignoring it! 😄

[swissspidy]

Would be great to see this change 👍

[TacoError]

it has been 2 years :(