build: add typescript as devDependencies

Author: alan-agius4

packages/ngtools/webpack/package.json

@@ -35,6 +35,7 @@
     "webpack": "^4.0.0"
   },
   "devDependencies": {
+    "typescript": "~3.0.1",
     "webpack": "^4.0.0"
   }
 }