add concurrently to package.json

package.json

@@ -12,6 +12,7 @@
   },
   "devDependencies": {
     "@types/node": "^14.14.31",
+    "concurrently": "^6.0.0",
     "sass": "^1.32.8",
     "typescript": "^4.2.2"
   }