fix(types): update interceptors type

[farnabaz]

Update return types of interceptor functions to match axios interceptor and module logic.
The interceptor implementation provide possibility to return config.

// https://github.com/nuxt-community/axios-module/blob/master/lib/plugin.js#L24
onRequest(fn) {
 this.interceptors.request.use(config => fn(config) || config)
},

Also Axios interceptors can support async callbacks. And reject handlers can return any type not just AxiosError

// https://github.com/axios/axios/blob/master/index.d.ts#L125
export interface AxiosInterceptorManager<V> {
  use(onFulfilled?: (value: V) => V | Promise<V>, onRejected?: (error: any) => any): number;
  eject(id: number): void;
}

export interface AxiosInstance {
  interceptors: {
    request: AxiosInterceptorManager<AxiosRequestConfig>;
    response: AxiosInterceptorManager<AxiosResponse>;
  };
}

resolves #441

[codecov]

Codecov Report

Merging #476 (9b4d043) into master (62f17ca) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #476   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files           1        1           
  Lines          51       51           
  Branches       27       27           
=======================================
  Hits           45       45           
  Misses          6        6           

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