Best Place for {code-cell} -- :load: option

[mmcky]

This myst-nb PR incorporates an experimental syntax to bring code in from a file using :load: in code-cell directives such as:

```{code-cell}
:load: <some-file.py>
```

There has been a lot of discussion focused on the "best" place to implement this feature as having it in myst_nb means it is not technically part of the core myst syntax -- but rather a feature to myst_nb + jupyter-book users. There are a couple of options to consider for migrating this from myst_nb in the future:

• jupytext with directive like support for metadata parsing of :load:. This is similar to how ipython magics work as the cell needs to be executed once to act on the action and then again to act on the code
• papermill (?)
• myst-nb extension

another option is to convert this to be a sphinx-extension to support myst as per this suggestion from @choldgraf

Though one thought for the long term, could we add an extension point to the myst nb sphinx extension ? Eg if you could register a function that would take a notebook as input, and then hook that up to either a pre or post execute callback, then this kind of functionality could just be a sphinx extension rather than supported natively by myst nb.

We could look at papermill for some inspiration https://papermill.readthedocs.io/en/latest/extending-overview.html

(Or to be honest I wonder if there is some way we could use papermill under the hood for these use cases? That might be more complex tho)