#### 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
id: number;
  name: string;
}
const handleUserResponse = (response: AxiosResponse<User>) => {
	console.log(response.data.id);
	console.log(response.data.name);
	console.log(response.status);
	console.log(response.statusText);
	console.log(response.headers);
	console.log(response.config);
};
axios.get<User>('/user?id=12345')
	.then(handleUserResponse)
	.catch(handleError);
axios.get<User>('/user', { params: { id: 12345 } })
	.then(handleUserResponse)
	.catch(handleError);
axios.head<User>('/user')
	.then(handleUserResponse)
    .catch(handleError);
axios.options<User>('/user')
	.then(handleUserResponse)
	.catch(handleError);
axios.delete<User>('/user')
	.then(handleUserResponse)
	.catch(handleError);
axios.post<User>('/user', { name: 'foo', id: 1 })
	.then(handleUserResponse)
	.catch(handleError);
axios.post<User>('/user', { name: 'foo', id: 1 }, { headers: { 'X-FOO': 'bar' } })