[Enhancement] Extension scopes

### Before submitting

- [x] I am using the latest version of Quarkdown.
- [x] I have checked the [wiki](https://quarkdown.com/wiki) to confirm this feature doesn't already exist.
- [x] I have searched existing [issues](https://github.com/iamgio/quarkdown/issues) for duplicates.

### Description

Should allow `.extend` in scopes to only act within that scope, including primitive extensions.

### Proposed syntax or behavior

```markdown
.if {yes}
.extend {heading}
.super foreground:{blue}

# Blue

# Not blue
```

### Additional context

_No response_

Start by tracing how `.extend` is parsed and applied within `.if` scopes, including primitive extensions. Use the proposed example as the first verification case: the heading inside the scope should be blue, while the heading after the scope should not be. Done means scoped extensions no longer affect content outside their scope.

Written by the indexing model from the issue text.