[Bug Report] Interface'NuxtApp'incorrectly extended interface'Vue

### Element UI version
2.12.0

### OS/Browsers version
macos/chrome

### Vue version
2.6.10

### Reproduction Link
https://github.com/iwata/codesandbox-nuxt

### Steps to reproduce
$ git clone git@github.com:iwata/codesandbox-nuxt.git
$ yarn install
$ yarn dev

### What is Expected?
不报错

### What is actually happening?
ERROR in /Users/iwata/.go/src/github.com/iwata/codesandbox-nuxt/node_modules/@nuxt/vue-app/types/index.d.ts(73,18):
TS2430: Interface 'NuxtApp' incorrectly extends interface 'Vue'.
Types of property '$loading' are incompatible.
Property 'open' is missing in type 'NuxtLoading' but required in type '{ open: (params: LoadingConfig) => { close: () => any; }; }'.