`CreateWixBuildWixpack` task does not support nested <?if?> and related preprocessor directives

Consider updating the task to support nesting of preprocessor directives.

I.e., the following doesn't work today:
```

```

Locate the CreateWixBuildWixpack task implementation and inspect how it handles the nested preprocessor example from the issue. Check for existing tests covering ifdef, if, and endif directives; done means the shown nested directives are processed successfully without breaking existing cases.

Written by the indexing model from the issue text.