Improve context collection

[Yahiewi]

This PR improves the context collection by traversing preceding cells and formatting student inputs into structured XML tags for the LLM.

[github-actions]

👈 Launch a Binder on branch Yahiewi/jupyter-ai-tutor/context

[brichet]

This PR will conflict with #16.
Let merge #16 first and finalize this one after.

[brichet]

Thanks @Yahiewi for working on this, it looks good overall.

We should probably add a short description about the tags in the default TUTOR.md (<student_context>, <student_answer>, <reference_solution> and <evaluation_criteria>)

[Yahiewi]

Thanks @brichet for the reviews. I just updated the PR to address all of them (aside from adding descriptions to the TUTOR.md as it's still not entirely clear how we should do that I think)

[brichet]

I added a short description of the content tags in 1cba91d

[brichet]

@Yahiewi if you want to take a look.

[Yahiewi]

This looks great @brichet thanks! I think I'll open a PR to add the initial_source tag like we discussed, then another PR to implement a first version of the "Review" command.