fix: add deprecated warn for tapables + simple backward compatibility

lib/core/nuxt.js

@@ -58,6 +58,26 @@ export default class Nuxt {
     return this
   }
 
+  plugin(name, fn) {
+    // eslint-disable-next-line no-console
+    console.error(`[warn] nuxt.plugin('${name}',..) is deprecated. Please use new hooks system.`)
+
+    // A tiny backward compatibility util
+    const hookMap = {
+      'ready': 'ready',
+      'close': 'close',
+      'listen': 'listen',
+      'built': 'build:done'
+    }
+
+    if (hookMap[name]) {
+      this.hook(hookMap[name], fn)
+    }
+
+    // Always return nuxt class which has plugin() for two level hooks
+    return this
+  }
+
   hook(name, fn) {
     if (!name || typeof fn !== 'function') {
       return