need to add postinstall script

Author: Bruce Richardson

package.json

@@ -7,6 +7,7 @@
   "module": "build/esm/index.js",
   "types": "build/esm/index.d.ts",
   "scripts": {
+    "postinstall": "npm run build",
     "clean": "rm -rf build && rm -rf node_modules && npm install",
     "build": "npm run build:cjs && npm run build:esm",
     "build:cjs": "tsc --target es5 --outDir build/cjs --module commonjs",