Add strict check on npm version

This change adds a strict check on npm version since npm v7 introduced
breaking changes related to the "preinstall" hook.

Ref: bb2c74f#commitcomment-52655924

Author: Ryan Sonshine

my-app/.npmrc

@@ -0,0 +1 @@
+engine-strict=true

my-app/package.json

@@ -3,6 +3,9 @@
   "version": "1.0.0",
   "description": "A sample application consuming a private scoped npm package",
   "main": "index.js",
+  "engines": {
+    "npm": "<=6"
+  },
   "scripts": {
     "preinstall": "npm run co:login",
     "co:login": "aws codeartifact login --tool npm --repository my-repo --domain my-domain"

my-package/.npmrc

@@ -0,0 +1 @@
+engine-strict=true

my-package/package.json

@@ -3,6 +3,9 @@
   "version": "1.0.0",
   "description": "A sample private scoped npm package",
   "main": "index.js",
+  "engines": {
+    "npm": "<=6"
+  },
   "scripts": {
     "prepare": "npm run co:login",
     "co:login": "aws codeartifact login --tool npm --repository my-repo --domain my-domain"