feat: inaccessible replace_class_syscall detector

### Describe the desired feature

I have a suggestion for a new detector based on a bug I've seen in one codebase.

A contract was supposed to be upgradable via the [`replace_class_syscall`](https://docs.starknet.io/documentation/architecture_and_concepts/Smart_Contracts/system-calls-cairo1/#replace_class), but the function wrapping this syscall was not exposed as an entrypoint, i.e. it was missing the `#[external(v0)]` attribute.

TBH I'm not sure how caracal works under the hood, but this feels like something it could detect 🤞

Start by reading Caracal's existing detector architecture and the Starknet replace_class_syscall documentation linked in the issue. Identify how the analyzer recognizes exposed entrypoints, then define the detector's done condition as reporting a wrapper around replace_class_syscall that lacks the required external entrypoint attribute.

Written by the indexing model from the issue text.