<!-- Click "Preview" for a more readable version --

Please read and follow the instructions before submitting an issue:

- Read all our documentation, especially the [README](https://github.com/axios/axios/blob/master/README.md). It may contain information that helps you solve your issue.
- Ensure your issue isn't already [reported](https://github.com/axios/axios/issues?utf8=%E2%9C%93&q=is%3Aissue).
- If you aren't sure that the issue is caused by Axios or you just need help, please use [Stack Overflow](https://stackoverflow.com/questions/tagged/axios) or [our chat](https://gitter.im/mzabriskie/axios).
- If you're reporting a bug, ensure it isn't already fixed in the latest Axios version.
- Don't remove any title of the issue template, or it will be treated as invalid by the bot.

⚠️👆 Feel free to these instructions before submitting the issue 👆⚠️
-->

#### 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.

Gruntfile.js
}
    },
    eslint: {
      target: ['lib/**/*.js']
    },
    karma: {
      options: {
        configFile: 'karma.conf.js'
      },
      single: {
        singleRun: true
      },
      continuous: {
        singleRun: false
      }
    },
    mochaTest: {
      test: {
        src: ['test/unit/**/*.js']
      },
      options: {
        timeout: 30000
      }
    },
    watch: {
      build: {
        files: ['lib/**/*.js'],
        tasks: ['build']
      },
      test: {