BUG: Can't include nested Headings in Directives

[mmcky]

Describe the bug

You can't have nested headings within exercise directives such as

```{exercise-start}
:label:
```

## Heading 2

```{exercise-end}
```

this causes the nodes not be correctly collected and parsed in the AST

Reproduce the bug

See above example

List your environment

No response