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#### Describe the issue
Headers are typed as `Record<string, string>`. This includes `axios.defaults.headers`, which is configured as: https://github.com/axios/axios/blob/76f09afc03fbcf392d31ce88448246bcd4f91f8c/lib/defaults.js#L119-L123

This seems incompatible with `headers` being of type `Record<string, string>`.

#### Example Code
The following line produces a TypeScript error beacuse `headers.commons` is of type `string`: "Element implicitly has an 'any' type because index expression is not of type 'number'.ts(7015)".

```ts
axios.defaults.headers.common['Authorization'] = AUTH_TOKEN
```


#### Expected behavior, if applicable
I expect to be able to configure default headers without TypeScript errors. Using the code examples from the Axios documentation, I would exspect `axios.defaults.headers.common` to be of type `Record<string, string>` not of type `string`.

#### Environment
 - Axios Version 0.22.0 
 - Adapter N/A
 - Browser N/A
 - Browser Version N/A
 - Node.js Version 15.14.0
 - OS: Ubuntu 21.04
 - Additional Library Versions TypeScript 4.4.3

#### Additional context/Screenshots
Add any other context about the problem here. If applicable, add screenshots to help explain.

index.d.ts
t?: any;
  httpsAgent?: any;
  proxy?: AxiosProxyConfig | false;
  cancelToken?: CancelToken;
  decompress?: boolean;
  transitional?: TransitionalOptions;
  signal?: AbortSignal;
  insecureHTTPParser?: boolean
}
export interface AxiosResponse<T = unknown, D = any>  {
  data: T;
  status: number;
  statusText: string;
  headers: AxiosResponseHeaders;
  config: AxiosRequestConfig<D>;
  request?: any;
}
export interface AxiosError<T = unknown, D = any> extends Error {
  config: AxiosRequestConfig<D>;
  code?: string;
  request?: any;
  response?: AxiosResponse<T, D>;
  isAxiosError: boolean;
  toJSON: () => object;
}
export interface AxiosPromise<T = unknown> extends Promise<AxiosResponse<T>> {
}
export interface CancelStatic {
  new (message?: string): Cancel;
}
export interface Cancel {
  message: string;
}
export interface Canceler {
  (message?: string): void;
}
export interface CancelTokenStatic {
  new (executor: (cancel: Canceler) => void): CancelToken;
  source(): CancelTokenSource;
}
export interface CancelToken {
  promise: Promise<Cancel>;
  reason?: Cancel;
  throwIfRequested(): void;
}
export interface CancelTokenSource {
  token: CancelToken;
  cancel: Canceler;
}
export interface AxiosInterceptorManager<V> {
  use<T = V>(onFulfilled?: (value: V) => T | Promise<T>, onRejected?: (error: any) => any): number;
  eject(id: number): void;
}
export class Axios {
  constructor(config?: AxiosRequestConfig);
  defaults: AxiosRequestConfig;
  interceptors: {
    request: AxiosInterceptorManager<AxiosRequestConfig>;
    response: AxiosInterceptorManager<AxiosResponse>;
  };
  getUri(config?: AxiosRequestConfig): string;
  request<T = unknown, R = AxiosResponse<T>, D = any> (config: AxiosRequestConfig<D>): Promise<R>;
  get<T = unknown, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  delete<T = unknown, R = AxiosResponse<T>, D = any>(url: string, config?: AxiosRequestConfig<D>): Promise<R>;
  head<T = unknown, R = AxiosResp