[BUG] PUBSUB SHARDNUMSUB doesn't return -CROSSSLOT for cross slot shard-channels

Describe the bug

In cluster mode, calling this command with two channels mapping to different slots, like PUBSUB SHARDNUMSUB ch1 ch2, succeeds. It should return a -CROSSSLOT error, like any command with keys of two different slots.

To reproduce

Call the command with two channels mapping to the same node, but different cluster slots.

Expected behavior

-CROSSSLOT error reply.

Additional information

I believe it's enough to add a key specification in the command's JSON file under src/commands/. That would make the generic machinery for cluster crosslot detect the error early and return an error.