keep-ipynb notebook with cell magics and yaml options can't be executed

e.g.

```{python}
#| output: asis
%%javascript

// some JS code
```

This is because cell magics need to be the first line of the cell.

Note that the keep-ipynb feature is really designed for debugging rather than for producing a final notebook for distribution (e.g. markdown constructs like citations and crossrefs are not resolved in this version). We should either resolve this or make it more clear in the documentation that keep-ipynb is for debugging intermediate representations (we could even change the filename to make this more clear).

Start with the keep-ipynb feature and reproduce the supplied notebook containing YAML cell options and a JavaScript cell magic. Determine whether the generated notebook can execute with the cell magic first, or whether the documentation must clearly describe keep-ipynb as an intermediate debugging representation. Done means the behavior is resolved or the limitation is clearly documented.