Switch from "fs" to XMLHttpRequest

As @Kireus accurately reported, the plugin worked just fine for playtesting, but on deployment, would simply vanish.

The underlying reason for this was that the code relied on "fs", a Node.js module, being present. MV, when playtesting, emulates a Node environment in the browser (using NW.js), but for some reason, doesn't when the project is deployed (or maybe I did something else wrong, who knows?)

To solve this, I switched to what DataManager uses: XMLHttpRequest. I only needed to change how the configuration file was loaded, so it should work the same as before, but should now also work when you deploy your project!