Add Webpack as devDependency

This is now used by the Flight package so it needs to be installed as a
peer.

Author: sebmarkbage

package.json

@@ -87,7 +87,8 @@
     "targz": "^1.0.1",
     "through2": "^2.0.0",
     "tmp": "~0.0.28",
-    "typescript": "~1.8.10"
+    "typescript": "~1.8.10",
+    "webpack": "^4.41.2"
   },
   "devEngines": {
     "node": "8.x || 9.x || 10.x || 11.x || 12.x"