[New feature] Step-by-step exercises

Allow for a specific task to be split into multiple subtasks representing the different steps of the initial problem.
Current solution: A link in the context, allowing the student to have access to another task with more subproblems representing the different steps.

The issue does not name files, tests, or an entry point. Start by reviewing how the current context link opens another task with subproblems, then determine how a single task should represent multiple subtasks. Done means the proposed step-by-step exercise flow is implemented and verified for a task with several ordered subtasks.

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