Publish only build artifacts

This a bit reduce amount of published files and final package size.

Author: TrySound

.npmignore

@@ -10,6 +10,10 @@
   },
   "main": "lib/index.js",
   "module": "esm/index.js",
+  "files": [
+    "lib",
+    "esm"
+  ],
   "scripts": {
     "build": "babel src --out-dir lib && NODE_ENV=esm babel src --out-dir esm",
     "prepublish": "npm run build",