DEV: Switch to use pnpm (#6)

.npmrc

+engine-strict = true
+auto-install-peers = false

package.json

 {
   "private": true,
   "devDependencies": {
-    "@discourse/lint-configs": "1.3.9",
+    "@discourse/lint-configs": "1.3.10",
     "ember-template-lint": "6.0.0",
-    "eslint": "8.57.0",
-    "prettier": "2.8.8"
+    "eslint": "8.57.1",
+    "prettier": "2.8.8",
+    "@babel/plugin-proposal-decorators": "^7.25.7"
+  },
+  "engines": {
+    "node": ">= 18",
+    "npm": "please-use-pnpm",
+    "yarn": "please-use-pnpm",
+    "pnpm": ">= 9"
   }
 }