#### 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((value: string) => {});
axios.get('/user')
  .then((response: AxiosResponse) => 'foo', (error: any) => 'bar')
  .then((value: string) => {});
axios.get('/user')
  .then((response: AxiosResponse) => 'foo', (error: any) => 123)
  .then((value: string | number) => {});
axios.get('/user')
  .catch((error: any) => 'foo')
  .then((value) => {});
axios.get('/user')
  .catch((error: any) => Promise.resolve('foo'))
  .then((value) => {});
const source: CancelTokenSource = axios.CancelToken.source();
axios.get('/user', {
  cancelToken: source.token
}).catch((thrown: AxiosError | Cancel) => {
  if (axios.isCancel(thrown)) {
    const cancel: Cancel = thrown;
    console.log(cancel.message);
  }
});
source.cancel('Operation has been canceled.');
axios.get('/user')
  .catch((error) => {
    if (axios.isAxiosError(error)) {
      const axiosError: AxiosError = error;
    }
  });