#### 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/specs/api.spec.js
describe('static api', function () {
  it('should have request method helpers', function () {
    expect(typeof axios.request).toEqual('function');
    expect(typeof axios.get).toEqual('function');
    expect(typeof axios.head).toEqual('function');
    expect(typeof axios.options).toEqual('function');
    expect(typeof axios.delete).toEqual('function');
    expect(typeof axios.post).toEqual('function');
    expect(typeof axios.put).toEqual('function');
    expect(typeof axios.patch).toEqual('function');
  });
  it('should have promise method helpers', function () {
    var promise = axios();
    expect(typeof promise.then).toEqual('function');
    expect(typeof promise.catch).toEqual('function');
  });
  it('should have defaults', function () {
    expect(typeof axios.defaults).toEqual('object');
    expect(typeof axios.defaults.headers).toEqual('object');
  });
  it('should have interceptors', function () {
    expect(typeof axios.interceptors.request).toEqual('object');
    expect(typeof axios.interceptors.response).toEqual('object');
  });
  it('should have all/spread helpers', function () {