compat: add a step on typescript typing alias

Author: yyx990803

src/guide/migration/migration-build.md

@@ -155,11 +155,22 @@ The following workflow walks through the steps of migrating an actual Vue 2 app
 
    </details>
 
-4. At this point, your application may encounter some compile-time errors / warnings (e.g. use of filters). Fix them first. If all compiler warnings are gone, you can also set the compiler to Vue 3 mode.
+4. If you are using TypeScript, you will also need to modify `vue`'s typing to expose the default export (which is no longer present in Vue 3) by adding a `*.d.ts` file with the following:
+
+   ```ts
+   declare module 'vue' {
+     import { CompatVue } from '@vue/runtime-dom'
+     const Vue: CompatVue
+     export default Vue
+     export * from '@vue/runtime-dom'
+   }
+   ```
+
+5. At this point, your application may encounter some compile-time errors / warnings (e.g. use of filters). Fix them first. If all compiler warnings are gone, you can also set the compiler to Vue 3 mode.
 
    [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/b05d9555f6e115dea7016d7e5a1a80e8f825be52)
 
-5. After fixing the errors, the app should be able to run if it is not subject to the [limitations](#known-limitations) mentioned above.
+6. After fixing the errors, the app should be able to run if it is not subject to the [limitations](#known-limitations) mentioned above.
 
    You will likely see a LOT of warnings from both the command line and the browser console. Here are some general tips:
 
@@ -171,29 +182,29 @@ The following workflow walks through the steps of migrating an actual Vue 2 app
 
    - If you are using `vue-router`, note `<transition>` and `<keep-alive>` will not work with `<router-view>` until you upgrade to `vue-router` v4.
 
-6. Update [`<transition>` class names](/guide/migration/transition.html). This is the only feature that does not have a runtime warning. You can do a project-wide search for `.*-enter` and `.*-leave` CSS class names.
+7. Update [`<transition>` class names](/guide/migration/transition.html). This is the only feature that does not have a runtime warning. You can do a project-wide search for `.*-enter` and `.*-leave` CSS class names.
 
    [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/d300103ba622ae26ac26a82cd688e0f70b6c1d8f)
 
-7. Update app entry to use [new global mounting API](/guide/migration/global-api.html#a-new-global-api-createapp).
+8. Update app entry to use [new global mounting API](/guide/migration/global-api.html#a-new-global-api-createapp).
 
    [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/a6e0c9ac7b1f4131908a4b1e43641f608593f714)
 
-8. [Upgrade `vuex` to v4](https://next.vuex.vuejs.org/guide/migrating-to-4-0-from-3-x.html).
+9. [Upgrade `vuex` to v4](https://next.vuex.vuejs.org/guide/migrating-to-4-0-from-3-x.html).
 
    [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/5bfd4c61ee50f358cd5daebaa584f2c3f91e0205)
 
-9. [Upgrade `vue-router` to v4](https://next.router.vuejs.org/guide/migration/index.html). If you also use `vuex-router-sync`, you can replace it with a store getter.
+10. [Upgrade `vue-router` to v4](https://next.router.vuejs.org/guide/migration/index.html). If you also use `vuex-router-sync`, you can replace it with a store getter.
 
-   After the upgrade, to use `<transition>` and `<keep-alive>` with `<router-view>` requires using the new [scoped-slot based syntax](https://next.router.vuejs.org/guide/migration/index.html#router-view-keep-alive-and-transition).
+    After the upgrade, to use `<transition>` and `<keep-alive>` with `<router-view>` requires using the new [scoped-slot based syntax](https://next.router.vuejs.org/guide/migration/index.html#router-view-keep-alive-and-transition).
 
-   [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/758961e73ac4089890079d4ce14996741cf9344b)
+    [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/758961e73ac4089890079d4ce14996741cf9344b)
 
-10. Pick off individual warnings. Note some features have conflicting behavior between Vue 2 and Vue 3 - for example, the render function API, or the functional component vs. async component change. To migrate to Vue 3 API without affecting the rest of the application, you can opt-in to Vue 3 behavior on a per-component basis with the [`compatConfig` option](#per-component-config).
+11. Pick off individual warnings. Note some features have conflicting behavior between Vue 2 and Vue 3 - for example, the render function API, or the functional component vs. async component change. To migrate to Vue 3 API without affecting the rest of the application, you can opt-in to Vue 3 behavior on a per-component basis with the [`compatConfig` option](#per-component-config).
 
     [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/d0c7d3ae789be71b8fd56ce79cb4cb1f921f893b)
 
-11. When all warnings are fixed, you can remove the migration build and switch to Vue 3 proper. Note you may not be able to do so if you still have dependencies that rely on Vue 2 behavior.
+12. When all warnings are fixed, you can remove the migration build and switch to Vue 3 proper. Note you may not be able to do so if you still have dependencies that rely on Vue 2 behavior.
 
     [Example commit](https://github.com/vuejs/vue-hackernews-2.0/commit/9beb45490bc5f938c9e87b4ac1357cfb799565bd)