workflow: alert user working on repo if not using yarn (#1237)

package.json

@@ -16,7 +16,8 @@
     "changelog": "conventional-changelog -p angular -i CHANGELOG.md -s",
     "dev-compiler": "npm-run-all --parallel \"dev template-explorer\" serve",
     "serve": "serve",
-    "open": "open http://localhost:5000/packages/template-explorer/local.html"
+    "open": "open http://localhost:5000/packages/template-explorer/local.html",
+    "preinstall": "node ./scripts/checkYarn.js"
   },
   "types": "test-dts/index.d.ts",
   "tsd": {

scripts/checkYarn.js

@@ -0,0 +1,5 @@
+if (!/yarn\.js$/.test(process.env.npm_execpath || '')) {
+  console.warn(
+    "\u001b[33mYou don't seem to be using yarn. This could produce unexpected results.\u001b[39m"
+  )
+}