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.

build/build.js

@@ -1,20 +1,23 @@
-const minify = require('@node-minify/core');
-const uglify = require('@node-minify/uglify-es');
+var fs = require('fs');
+const { resolve } = require('path');
 
-minify({
-  compressor: uglify,
-  input: './src/color-thief.js',
-  output: './dist/color-thief.min.js',
-  options: {
-    output: {
-        comments: 'some'
-    }
-  },
-  callback: function(err, min) {
-    if (err) {
-        console.log('⚠️ERROR:' + err);
-    } else {
-        console.log('✅ Minification completed');
-    }
-  }
+/*
+color-thief.umd.js duplicated as color-thief.min.js for legacy support
+
+In Color Thief v2.1 <= there was one distribution file (dist/color-thief.min.js)
+and it exposed a global variable ColorThief. Starting from v2.2, the package
+includes multiple dist files for the various module systems. One of these is
+the UMD format which falls back to a global variable if the requirejs AMD format
+is not being used. This file is called color-thief.umd.js in the dist folder. We
+want to keep supporting the previous users who were loading
+dist/color-thief.min.js and expecting a global var. For this reason we're
+duplicating the UMD compatible file and giving it that name.
+*/
+
+const source = resolve(process.cwd(), 'dist/color-thief.umd.js');
+const dest = resolve(process.cwd(), 'dist/color-thief.min.js');
+
+fs.copyFile(source, dest, (err) => {
+    if (err) throw err;
+    console.log('source.txt was copied to destination.txt');
 });