Feature: add support for .py files in plaintext formats

Add support for exporting the following plaintext formats as if they were regular `.ipynb` files:

- `percent`
- `light`
- `sphinx`
- `myst`
- `pandoc`

See [this](https://jupytext.readthedocs.io/en/latest/formats-scripts.html) for details on the above plain-text notebook formats. This feature is inspired by [this](https://fgiasson.com/blog/index.php/2023/08/30/literate-programming-in-python-using-nbdev/) article, where the author also requested it.

(Note: This is a reissue of a previous poorly attempted [pull request](https://github.com/AnswerDotAI/nbdev/pull/1436))