feat: add repeated structures

[mstechly]

Description

Adds support for repeated structures and a couple of other minor features.
For tests to pass it would require PsiQ/qref#124 to go in first an QREF to be released.

There's a couple of things left:

• Documentation
• Some minor test cases are missing
• Moving _evaluate_and_define_functions in an appropriate place.

Please verify that you have completed the following steps

• I have self-reviewed my code.
• I have included test cases validating introduced feature/fix.
• I have updated documentation.

[dexter2206]

Overall looks good, and I only left minor suggestions, with a following caveat:

After reviewing this and the QREF's counterpart of this PR I'm more and more convinced that it would be better if we had a separate class representing "loop" or a "routine with repetitions" (or at least if we had it in Bartiq). This is because a lot of the logic we perform for validation and dispatching (with ifs) could be neatly hidden. For instance, we verify if routine with repetition has one child - but we could have an object that HAS to have one child (which would be validated by Pydantic). Anyway, we can continue with the current solution and later refactor it if we have some time to do so.