#### Summary

It would be nice to export a typeguard from Axios' index.d.ts.
This way, consumers trying to do error handling can check in a type-safe way if the error is an AxiosError, or some other kind of error.

It's easy enough to do this as a consumer, but it would be nicer if this was included in the actual Axios library.

test/typescript/axios.ts
.catch(handleError);
instance1.options('/user')
  .then(handleResponse)
  .catch(handleError);
instance1.get('/user', { params: { id: 12345 } })
  .then(handleResponse)
  .catch(handleError);
instance1.post('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
instance1.post('/user', { foo: 'bar' }, { headers: { 'X-FOO': 'bar' } })
  .then(handleResponse)
  .catch(handleError);
axios.defaults.baseURL = 'https://api.example.com/';
axios.defaults.headers.common['Authorization'] = 'token';
axios.defaults.headers.post['X-FOO'] = 'bar';
axios.defaults.timeout = 2500;
instance1.defaults.baseURL = 'https://api.example.com/';
instance1.defaults.headers.common['Authorization'] = 'token';
instance1.defaults.headers.post['X-FOO'] = 'bar';
instance1.defaults.timeout = 2500;
const requestInterceptorId: number = axios.interceptors.request.use(
  (config: AxiosRequestConfig) => config,
  (error: any) => Promise.reject(error)
);
axios.interceptors.request.eject(requestInterceptorId);
axios.interceptors.request.use(
  (config: AxiosRequestConfig) => Promise.resolve(config),
  (error: any) => Promise.reject(error)
);