chore: add core-js@3 example in readme (#5633)

packages/babel-preset-app/README.md

@@ -1,21 +1,11 @@
 # `@nuxt/babel-preset-app`
+
 > Default babel preset for nuxt
 
 ## Usage
 
 This is the default preset used by Nuxt, which is mainly a wrapper around the `@babel/preset-env` preset. It also optionally uses the `@vue/babel-preset-jsx` preset as well as `@babel/plugin-syntax-dynamic-import`, `@babel/plugin-proposal-decorators`, `@babel/plugin-proposal-class-properties`, `@babel/plugin-transform-runtime`. Furthermore the preset is adding polyfills.
 
-Usually, no additional configuration is required. If needed though, there is an option to fine-tune the preset's behavior. Just add the following to `nuxt.config.js`:
-```js
-babel: {
-  presets({ isServer }) {
-    return [
-      [ "@nuxt/babel-preset-app", options ]
-    ]
-  }
-}
-```
-
 **Note**: Since `core-js@2` and `core-js@3` are both supported from Babel 7.4.0, we recommend directly adding `core-js` and setting the version via the [`corejs`](#corejs) option.
 
 ```sh
@@ -27,15 +17,30 @@ yarn add --dev core-js@2 @babel/runtime-corejs2
 
 ```
 
-...where `options` is an object with parameters, for example:
+Usually, no additional configuration is required. If needed though, there is an option to fine-tune the preset's behavior. Just add the following to `nuxt.config.js`:
+
+```js
+babel: {
+  presets({ isServer }) {
+    return [
+      [ "@nuxt/babel-preset-app", options ]
+    ]
+  }
+}
 ```
- const options = {
+
+`options` is an object with parameters, for example:
+
+```js
+const options = {
   useBuiltIns: "entry"
 }
 ```
+
 Below is a list of all available parameters:
 
 ### Options
+
 * **buildTarget** - passed in through the Builder, either `"server"` or `"client"`
 * **configPath** - [`@babel/preset-env` parameter](https://babeljs.io/docs/en/babel-preset-env#configpath)
 * **forceAllTransforms** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#forcealltransforms)' parameter
@@ -54,23 +59,57 @@ Below is a list of all available parameters:
 * **spec** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#spec)' parameter
 * **targets** - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#targets)' parameter
 * **useBuiltIns**, default `"usage"` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#usebuiltins)' parameter
+* **corejs**, default `{ version: 2 }` - '[@babel/preset-env](https://babeljs.io/docs/en/babel-preset-env#corejs)' parameter
 
 There are [detailed docs](https://babeljs.io/docs/en/babel-preset-env#options) for the parameters of '@babel/preset-env'.
 
 ### Example 1. Change targets for server and client respectively
+
 ```js
-babel: {
-  presets({ isServer }) {
-    return [
-      [
-        "@nuxt/babel-preset-app",
-        {
-          targets: isServer
-            ? { node: "current" }
-            : { browsers: ["last 2 versions"], ie: 11 }
-        }
-      ]
-    ]
+export default {
+  build: {
+    babel: {
+      presets({ isServer }) {
+        return [
+          [
+            "@nuxt/babel-preset-app",
+            {
+              targets: isServer
+                ? { node: "current" }
+                : { browsers: ["last 2 versions"], ie: 11 }
+            }
+          ]
+        ]
+      }
+    }
   }
-},
+}
+```
+
+### Example 2. Use core-js@3
+
+**NOTE**: Make sure that all dependencies have been upgraded to use core-js@3. If core-js@2 and core-js@3 are both dependent, babel may resolve incorrect core-js package which is hoisted by yarn/npm.
+
+```sh
+yarn add --dev core-js@2 @babel/runtime-corejs2
+```
+
+```js
+export default {
+  build: {
+    babel: {
+      presets({ isServer }) {
+        return [
+          [
+            require.resolve('@nuxt/babel-preset-app'),
+            {
+              buildTarget: isServer ? 'server' : 'client'
+              corejs: { version: 3 }
+            }
+          ]
+        ]
+      }
+    }
+  }
+}
 ```