Add option to log to Stackdriver

When the environment variable STACKDRIVER_LOGGING is set to true, logs
are sent to Stackdriver in addition to stdout.

libraries/logger/package.json

@@ -15,10 +15,12 @@
     "lint": "eslint -f unix ."
   },
   "dependencies": {
+    "@google-cloud/logging-bunyan": "^2.0.0",
     "@overleaf/o-error": "^2.0.0",
     "bunyan": "1.8.12",
     "raven": "1.1.3",
-    "request": "2.88.0"
+    "request": "2.88.0",
+    "yn": "^3.1.1"
   },
   "devDependencies": {
     "chai": "4.2.0",