Add typescript as a devDependency

We are using dtslint to test our type definitions which needs
typescript as a peer dependency.

Author: Tony D'Addeo

src/userbase-js/package-lock.json

@@ -45,6 +45,7 @@
     "optimize-css-assets-webpack-plugin": "^5.0.3",
     "style-loader": "^1.0.0",
     "terser-webpack-plugin": "^2.2.1",
+    "typescript": "^3.7.3",
     "webpack-node-externals": "^1.7.2"
   }
 }