use .qmd as as full replacement of .ipynb

hello!

wanted to know if it's possible to use only .qmd files for a full nbdev project (no .ipynb, all the code on .qmd) using nbdev's directives like so:

```

```{python}
#| default_exp core

\```

```{python}
#| hide
from nbdev.showdoc import *

\```

```{python}
#| export
def foo(): pass

\```

```{python}
#| hide
import nbdev; nbdev.nbdev_export()

\```

```

and get a result package with a `core.py` containing `foo()`

cheers,
Francis

Start at the nbdev.nbdev_export entry point and trace how the shown directives are processed for .ipynb inputs. Assess the requested all-.qmd workflow, including the default_exp, hide, and export directives; done means a project with no .ipynb files produces a package whose core.py contains foo().

Written by the indexing model from the issue text.