#### 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
} from 'axios';
const config: AxiosRequestConfig = {
  url: '/user',
  method: 'get',
  baseURL: 'https://api.example.com/',
  transformRequest: (data: any) => '{"foo":"bar"}',
  transformResponse: [
    (data: any) => ({ baz: 'qux' })
  ],
  headers: { 'X-FOO': 'bar' },
  params: { id: 12345 },
  paramsSerializer: (params: any) => 'id=12345',
  data: { foo: 'bar' },
  timeout: 10000,
  withCredentials: true,
  auth: {
    username: 'janedoe',
    password: 's00pers3cret'
  },
  responseType: 'json',
  xsrfCookieName: 'XSRF-TOKEN',
  xsrfHeaderName: 'X-XSRF-TOKEN',
  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',