Add a requirement checking that certain functions are defined and warning students if not before running

There was a report on L5.P2 where the student has to code a function named `draw_three_squares()` and they used the number 3 instead in the name. There are a bunch of exercises like these where we need them to create a function or define a specific symbol.

We should have a general purpose library function we can use as a pre-requisite check for practices. We could then use it with a generic help message from relevant practices to tell students when they need to define a function named in a specific way.

In general, I'd like to bring in the same notion of requirements as we have in GDPractice.

This should be implemented first and then applied to relevant practices.

Start by finding how practices are defined and where pre-run checks occur; compare that flow with the requirement concept in GDPractice. Done means a reusable requirement check exists and relevant practices use it to warn about missing function or symbol definitions before execution.

Written by the indexing model from the issue text.