build: add reminder to keep engines in sync (#31266)

PR Close #31266

aio/package.json

@@ -77,6 +77,7 @@
     "post~~build": "yarn build-404-page",
     "~~light-server": "light-server --bind=localhost --historyindex=/index.html --no-reload"
   },
+  "//engines-comment": "Keep this in sync with ../package.json",
   "engines": {
     "node": ">=10.9.0 <11.0.0",
     "yarn": ">=1.12.1 <=1.16.0"

package.json

@@ -6,6 +6,7 @@
   "homepage": "https://github.com/angular/angular",
   "bugs": "https://github.com/angular/angular/issues",
   "license": "MIT",
+  "//engines-comment": "Keep this in sync with aio/package.json",
   "engines": {
     "node": ">=10.9.0 <11.0.0",
     "yarn": ">=1.12.1 <=1.16.0"