#### Is your feature request related to a problem? Please describe.
I want to use new toFormData method, but it is not typed in index.d.ts

#### Describe the solution you'd like
Add types for toFormData

#### Describe alternatives you've considered
Use ts-ignore

#### Additional context
None

test/typescript/axios.ts
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);
  } else {
    console.log(error.message);
  }
};
axios(config)
  .then(handleResponse)
  .catch(handleError);
axios.get('/user?id=12345')