#### 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
onUploadProgress: (progressEvent: ProgressEvent) => {},
  onDownloadProgress: (progressEvent: ProgressEvent) => {},
  maxContentLength: 2000,
  maxBodyLength: 2000,
  validateStatus: (status: number) => status >= 200 && status < 300,
  maxRedirects: 5,
  proxy: {
    host: '127.0.0.1',
    port: 9000
  },
  cancelToken: new axios.CancelToken((cancel: Canceler) => {})
};
const nullValidateStatusConfig: AxiosRequestConfig = {
  validateStatus: null
};
const undefinedValidateStatusConfig: AxiosRequestConfig = {
  validateStatus: undefined
};
const handleResponse = (response: AxiosResponse) => {
  console.log(response.data);
  console.log(response.status);
  console.log(response.statusText);
  console.log(response.headers);
  console.log(response.config);
};
const handleError = (error: AxiosError) => {
  if (error.response) {
    console.log(error.response.data);
    console.log(error.response.status);
    console.log(error.response.headers);