Indented checklists get flattened on preview

Indented checklists do not get properly converted to nested checklist items on any platform except web/Electron 😭

Tested this one on version 1.12.0.

### Expected
Indentation should be converted to nested list items, whether a checklist or a normal list.

Reported in #3182362-zen

### Observed
Nested checklists get flattened.

### Reproduced

## Test content:

```
- [ ] top level

- [X] indented item

- [ ] indented again
```

## Preview in iOS:

Issue opened here: https://github.com/Automattic/simplenote-ios/issues/827


## Preview in OSX:

## Preview in web/Electron:

## Preview on published note

(I will open a separate backend issue for this)

Start by reproducing the macOS preview with the checklist content shown in the issue, then trace the preview conversion path to find where indentation is flattened. Done means indented checklist and normal list items render as nested list items in the macOS preview, matching the web/Electron result.

Written by the indexing model from the issue text.