add post-install script

Author: Hweinstock

src.gen/@amzn/amazon-q-developer-streaming-client/package-lock.json

@@ -10,7 +10,8 @@
         "build:types": "tsc -p tsconfig.types.json",
         "build:types:downlevel": "downlevel-dts dist-types dist-types/ts3.4",
         "clean": "rimraf ./dist-* && rimraf *.tsbuildinfo",
-        "prepack": "npm run clean && npm run build"
+        "prepack": "npm run clean && npm run build",
+        "postinstall": "npm run build"
     },
     "main": "./dist-cjs/index.js",
     "types": "./dist-types/index.d.ts",