shared memory: enable shared memory in differential_v8 fuzz target

In the `differential_v8` fuzz target, the oracle retrieves the first exported memory and checks that the memory contents are the same between Wasmtime and V8. To properly enable shared memory in this fuzz target, we would need to check whether the first exported memory is shared and if so:
- use `get_shared_memory()` instead of `get_memory()` in the Wasmtime API
- use `SharedArrayBuffer` instead of `ArrayBuffer` in the V8 API