Migrate from CommonJS to ESM

Author: Kocal

package.json

@@ -2,7 +2,8 @@
   "name": "@symfony/webpack-encore",
   "version": "5.0.1",
   "description": "Webpack Encore is a simpler way to integrate Webpack into your application",
-  "main": "index.js",
+  "type": "module",
+  "exports": "./index.js",
   "scripts": {
     "test": "yarn run test:main && yarn run test:persistent-cache",
     "test:main": "mocha --reporter spec test --recursive --ignore test/persistent-cache/*",