#### 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

index.d.ts
RequestConfig<D>): Promise<R>;
  options<T = any, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  post<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  put<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  patch<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  postForm<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  putForm<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
  patchForm<T = any, R = AxiosResponse<T>, D = any>(url: string, data?: D, config?: AxiosRequestConfig<D>): Promise<R>;
}
export interface AxiosInstance extends Axios {
  (config: AxiosRequestConfig): AxiosPromise;
  (url: string, config?: AxiosRequestConfig): AxiosPromise;
}
export interface AxiosStatic extends AxiosInstance {
  create(config?: AxiosRequestConfig): AxiosInstance;
  Cancel: CancelStatic;
  CancelToken: CancelTokenStatic;
  Axios: typeof Axios;
  AxiosError: typeof AxiosError;
  readonly VERSION: string;
  isCancel(value: any): boolean;
  all<T>(values: Array<T | Promise<T>>): Promise<T[]>;
  spread<T, R>(callback: (...args: T[]) => R): (array: T[]) => R;
  isAxiosError(payload: any): payload is AxiosError;
}
declare const axios: AxiosStatic;
export default axios;
