IntCyoaCreator download progress indicator

<!-- Insert the following in the beginning of <body> (on the line above <div id="app">) -->
<div id="indicator"><script>{let _XHR = XMLHttpRequest;  XMLHttpRequest = class XHR extends _XHR {constructor () {
  super();
  this.addEventListener('progress', e => {indicator.innerText = " Loading data: " + (!e.total ? `${e.loaded} bytes` : `${(100 * e.loaded / e.total).toFixed(2)}%`)});
  this.addEventListener('loadend', () => {indicator.innerText = ""});
}}}</script></div>
<!-- Modifier: replace `${e.loaded} bytes` with `${(e.loaded/1024**2).toFixed(1)} MB` to display size in MB -->
<!-- Modifier: replace the part after " Loading data: " with `${(100 * e.loaded / (e.total||SIZE)).toFixed(2)}%` to
               always show percentage (SIZE is project.json size in bytes; remember to replace it on every update) -->