#### 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

export type AxiosRequestHeaders = Record<string, string | number | boolean>;
export type AxiosResponseHeaders = Record<string, string> & {
  "set-cookie"?: string[]
};
export interface AxiosRequestTransformer {
  (data: any, headers?: AxiosRequestHeaders): any;
}
export interface AxiosResponseTransformer {
  (data: any, headers?: AxiosResponseHeaders): any;
}
export interface AxiosAdapter {
  (config: AxiosRequestConfig): AxiosPromise;
}
export interface AxiosBasicCredentials {
  username: string;
  password: string;
}
export interface AxiosProxyConfig {
  host: string;
  port: number;
  auth?: {
    username: string;
    password: string;
  };
  protocol?: string;
}
export type Method =
  | 'get' | 'GET'
  | 'delete' | 'DELETE'
  | 'head' | 'HEAD'
  | 'options' | 'OPTIONS'
  | 'post' | 'POST'
  | 'put' | 'PUT'
  | 'patch' | 'PATCH'
  | 'purge' | 'PURGE'
  | 'link' | 'LINK'
  | 'unlink' | 'UNLINK';
export type ResponseType =
  | 'arraybuffer'
  | 'blob'
  | 'document'
  | 'json'
  | 'text'
  | 'stream';
  export type responseEncoding =
  | 'ascii' | 'ASCII'
  | 'ansi' | 'ANSI'
  | 'binary' | 'BINARY'
  | 'base64' | 'BASE64'
  | 'base64url' | 'BASE64URL'
  | 'hex' | 'HEX'
  | 'latin1' | 'LATIN1'
  | 'ucs-2' | 'UCS-2'
  | 'ucs2' | 'UCS2'
  | 'utf-8' | 'UTF-8'
  | 'utf8' | 'UTF8'
  | 'utf16le' | 'UTF16LE';
export interface TransitionalOptions {
  silentJSONParsing?: boolean;
  forcedJSONParsing?: boolean;
  clarifyTimeoutError?: boolean;
}
export interface AxiosRequestConfig<D = any> {
  url?: string;
  method?: Method | string;
  baseURL?: string;
  transformRequest?: AxiosRequestTransformer | AxiosRequestTransformer[];
  transformResponse?: AxiosResponseTransformer | AxiosResponseTransformer[];
  headers?: AxiosRequestHeaders;
  params?: any;
  paramsSerializer?: (params: any) => string;
  data?: D;
  timeout?: number;
  timeoutErrorMessage?: string;
  withCredentials?: boolean;
  adapter?: AxiosAdapter;
  auth?: AxiosBasicCredentials;
  responseType?: ResponseType;