[RFC]: add `fs/mkdir`

### Description

This RFC proposes adding the package `@stdlib/fs/mkdir` for creating a directory asynchronously and synchronously.

Package: `@stdlib/fs/mkdir`
```javascript
var mkdir = require( '@stdlib/fs/mkdir' );
var mkdirSync = require( '@stdlib/fs/mkdir' ).sync;
```

### Related Issues

None.

### Questions

No.

### Other

No.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.