himl flattens nested lists

Running `himl` on nested lists results in a flattened list. For example:

```yaml
test:
- [1, 2]
- [3, 4]
```

becomes the following after `himl` processes it.

```yaml
test:
- 1
- 2
- 3
- 4
```

I believe that `himl` should not modify the structure of this data.

Reproduce the issue using the nested-list YAML example in the report, then trace how himl processes lists. The work is done when nested lists retain their original structure instead of being flattened.

Written by the indexing model from the issue text.