chore(.nvmrc): update to 'lts/iron' for ESLint compatibility and add related comment (#226)

Author: sukvvon

.nvmrc

@@ -1 +1,5 @@
-v20
+lts/iron
+
+# 'iron' is the 20.x node release line
+# ESLint requires Node.js >=18.18.0, >=20.9.0, or >=21.1.0 — ensure 20.9.0+ if using iron
+# See: https://eslint.org/docs/latest/use/getting-started#prerequisites