gh-127833: Use `productionlist` nodes to implement the `grammar-snippet` directive by AA-Turner · Pull Request #130376 · python/cpython

Sorry for the delay on my part. I've refactored to split the parsing of content lines into individual productions into its own method, as I think it is sufficiently complex to warrant a dedicated function (and this massively simplifies the creation of the actual production node).

One open question is if group_dict is the right representation, as my understanding is that each dict must have exactly one key -- perhaps an enum would be a better fit? I'm not sure though, and we mustn't let perfect be the enemy of the good.

A