Add a prepublish step

Author: Kevin Scott

package.json

@@ -9,6 +9,7 @@
     "clean": "rimraf dist/*",
     "watch": "npm run build -- --watch",
     "build": "npm run clean && tsc --project tsconfig.json",
+    "prepublish": "npm run build",
     "test": "jest"
   },
   "keywords": [