[1/5] refactor: rename InferenceChecker to ConflictChecker

[EmirDe]

Renames the InferenceChecker trait, its implementations and the fields that hold them to ConflictChecker and conflict_checker(s). A checker decides whether a set of premises is a conflict of a rule, which the new name says.

• No behavioural change; the diff is the rename only.
• check_inference and RejectedInference keep their names: they still describe checking one inference.

Review order:

• the trait file in pumpkin-checking, renamed from inference_checker.rs to conflict_checker.rs;
• where checkers are stored and called in pumpkin-core (propagation/runtime_checkers.rs, engine/state.rs, checkers/store.rs);
• the implementations in pumpkin-core (nogoods, hypercube linear, reified);
• the implementations in pumpkin-propagators, one file per constraint;
• documentation.

Stack: 1/5, based on main.

[ImkoMarijnissen]

Overall, the rename looks good to me; I find InferenceChecker a bit more clear in what its purpose is, but I understand that in the context of conflict rules, this naming makes sense