Summary

Implement IfKeyNotInDictAssign rule (RUF060)

which suggest to use setdefault.

closes #7897
see also dosisod/refurb#349

Notes

In the implementation:

• the functions is_same_arg/is_same_key are implemented in a strange (syntactic) way. I think it's worth catching complex expressions here (espessially in arg case), but can't find the right machinery to structured comparison of it.
• the functions is_idempotent/is_cheap_to_evaluate are implemented in really simple and conservative way, and so have a lot of false negative. We can implement them by using walk_expr machinery instead (e.g. to find function call-s), but I do not have strong opinion here. What do you think, is additional code complexity worth it?

Test Plan

cargo test