Switch to uber/npm-shrinkwrap for npm-shrinkwrap.json generation

Instead of our homegrown grunt/shrinkwrap.js
Fixes #13483.
New npm-shrinkwrap.json is human-readable but still in a consistent/stable format.
Should aid debugging, produce better diffs, and be less prone to merge conflicts

test-infra/S3Cachefile.json

@@ -1,6 +1,6 @@
 {
     "npm-modules": {
-        "key": "./npm-shrinkwrap.canonical.json",
+        "key": "./npm-shrinkwrap.json",
         "cache": "../node_modules",
         "generate": "./uncached-npm-install.sh"
     },