fix(types): correct Vuex Store instance (nuxt-community#227)

Author: RomainLanz

types/index.d.ts

@@ -1,5 +1,6 @@
 import { AxiosError, AxiosInstance, AxiosRequestConfig, AxiosResponse } from 'axios'
 import Vue from 'vue'
+import './vuex'
 
 interface NuxtAxiosInstance extends AxiosInstance {
   $request<T = any>(config: AxiosRequestConfig): Promise<T>

types/vuex.d.ts

@@ -0,0 +1,7 @@
+import { NuxtAxiosInstance } from '.'
+
+declare module 'vuex' {
+  interface Store<S> {
+    $axios: NuxtAxiosInstance,
+  }
+}