feat: Add module support with new dist files

- `/dist/color-thief.umd.js`: UMD module. For simple script tag loading that exposes a global variable or for RequireJS AMD support.
- `/dist/color-thief.js`: CommonJS module. Entry point for Node.js and Browserify.
- `/dist/color-thief.mjs`: ES6 module. For modern browsers as well as Webpack and Rollup.
- `/dist/color-thief.min.js`: Duplicate of `/dist/color-thief.umd.js`. Kept around to maintain backwards compatibility.

examples/js/demo.js

@@ -13,7 +13,7 @@ document.getElementById('example-images').innerHTML = examplesHTML;
 // Once images are loaded, process them
 document.querySelectorAll('.image').forEach((image) => {
     const section = image.closest('.image-section');
-    if (this.complete) {
+    if (image.complete) {
         showColorsForImage(image, section);
     } else {
         image.addEventListener('load', function() {