Develop a structure in the `quiz` module (or otherwise) for storing quiz questions and anwers

[alarthast]

• See the parent issue for context. Builds on #2212.

• There are two essential components to store, a str for the question and an ehrQL dataset / frame / series that serves at the model answer.

• It might be helpful to store an additional component, which is some starting point syntax. (e.g. If we ask for the clinical_events table to be filtered, an imported clinical_events table should serve as a starting point).

[alarthast]

Progress is at aw/sandbox-quiz

• Builds on the quiz module in #2212, so any updates there ought to be pulled into here.
• I have had an attempt at creating a Question class and a questions dictionary of question numbers as keys and Question objects as values. This might not be the most optimal way of storing things.
• Question objects have a starting_point property for the syntax we would like the user to start with.
• In particular, we designate the variable they should assign their answer to. This is primarily because the Question class was created with marimo notebooks in mind - marimo cells auto-update, which means that it is much cleaner to have each question correspond to a unique answer variable, rather than updating a generic dataset or frame variable as the user progresses through the quiz. If we do not use marimo notebooks, dictating the answer variable name is less relevant.

Implementation in marimo: