[App Engine] TS : Adding the missing node type definition (#2068)

The Typescript example is actually not working.
When App Engine is running the `npm run gcp-build`

It will throw erros like this one : 

`Step` #6 - "builder": index.ts(17,21): error TS2580: Cannot find name 'process'. 

The package.json is missing the node Types definitions module. 

My pull request is fixing that. 

Best

appengine/typescript/package.json

@@ -23,6 +23,7 @@
     "typescript": "^4.0.0"
   },
   "devDependencies": {
+    "@types/node": "^14.14.7",
     "mocha": "^8.0.0",
     "wait-port": "^0.2.7",
     "@types/express": "^4.16.0",