feat: add ES module builds (#2215)

packages/netlify-cms-core/package.json

@@ -4,13 +4,15 @@
   "version": "2.9.1-beta.0",
   "repository": "https://github.com/netlify/netlify-cms/tree/master/packages/netlify-cms-core",
   "bugs": "https://github.com/netlify/netlify-cms/issues",
+  "module": "dist/esm/index.js",
   "main": "dist/netlify-cms-core.js",
   "files": [
     "src/",
     "dist/"
   ],
   "scripts": {
-    "build": "cross-env NODE_ENV=production webpack"
+    "build": "cross-env NODE_ENV=production webpack",
+    "build:esm": "cross-env NODE_ENV=esm babel src --out-dir dist/esm --ignore src/**/__tests__/* --root-mode upward"
   },
   "keywords": [
     "netlify",