#### 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
.then(handleResponse)
  .catch(handleError);
axios.get('/user', { params: { id: 12345 } })
  .then(handleResponse)
  .catch(handleError);
axios.head('/user')
  .then(handleResponse)
  .catch(handleError);
axios.options('/user')
  .then(handleResponse)
  .catch(handleError);
axios.delete('/user')
  .then(handleResponse)
  .catch(handleError);
axios.post('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
axios.post('/user', { foo: 'bar' }, { headers: { 'X-FOO': 'bar' } })
  .then(handleResponse)
  .catch(handleError);
axios.put('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
axios.patch('/user', { foo: 'bar' })
  .then(handleResponse)
  .catch(handleError);
interface UserCreationDef {
    name: string;
}
interface User {