build: add check when running npm install

Even though we're using `yarn` for everything, it's still easy to fall into the old habit of running `npm i` which could mess up the `node_modules` and generate a new lock file. These changes add a script that throws an error when trying to `npm install`, similar to angular/angular.

Author: crisbeto

package.json

@@ -24,7 +24,8 @@
     "api": "gulp api-docs",
     "breaking-changes": "gulp breaking-changes",
     "gulp": "gulp",
-    "stage-release": "bash ./tools/release/stage-release-bin.sh"
+    "stage-release": "bash ./tools/release/stage-release-bin.sh",
+    "preinstall": "node ./tools/npm/check-npm.js"
   },
   "version": "7.0.2",
   "requiredAngularVersion": ">=7.0.0",

tools/npm/check-npm.js

@@ -0,0 +1,13 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+if (process.env.npm_execpath.indexOf('yarn') === -1) {
+    console.error('Please use Yarn instead of NPM to install dependencies. ' +
+                  'See: https://yarnpkg.com/lang/en/docs/install/');
+    process.exit(1);
+}