Allow import/require in Player.js for modularity of user code

Originally mentioned in https://github.com/olistic/warriorjs/issues/39, it would be a great improvement to code organization if users like myself could split our code into modules (*.js and *.mjs) that could be loaded into the Player.js file (via import and/or require).

It doesn't appear that this is supported at the moment. That means that a lot of code ends up in Player.js. At various times while playing, I've been in the neighborhood of 250 lines.