Support creating index with default export

Currently, this plugin only supports creating an index with named exports. I need an index file with a default-exported object.

Example:

```js
import foo from './foo.js';
import bar from './bar.js';
import _3_x_recommended from './3-x-recommended.js'; // note unsafe variable name needs to be tweaked

export default {
foo,
bar,
'3-x-recommended': _3_x_recommended
};
```

I see this functionality is in a forked version of this tool: https://github.com/tjjfvi/create-index-better#modes

> // @create-index {"mode":"default{}"}
>
> import foo from './foo.js';
> import bar from './bar.js';
> export default { foo, bar };

Has it been considered for the original `create-index`? @tjjfvi would you consider upstreaming this functionality (and presumably any other improvements from the fork)?

Start by reviewing the existing index-generation behavior and the fork's documented "default{}" mode. Trace how named exports and unsafe imported names are handled, then define completion against the provided example: generate a default-exported object with valid local identifiers and the requested property names.

Written by the indexing model from the issue text.