Prevent / being used in bot or server prefixes (#5693)

* Update `[p]set prefix`/`[p]set serverprefix`

* Update cli

* style

* update __main__

* style

* improve checks

* Raise in Red.set_prefixes, update responses

* uniform responses

* Fixes

* Keep generator variable names consistent across files

redbot/core/settings_caches.py

@@ -45,6 +45,10 @@ class PrefixManager:
         prefixes = prefixes or []
         if not isinstance(prefixes, list) and not all(isinstance(pfx, str) for pfx in prefixes):
             raise TypeError("Prefixes must be a list of strings")
+        if any(prefix.startswith("/") for prefix in prefixes):
+            raise ValueError(
+                "Prefixes cannot start with '/', as it conflicts with Discord's slash commands."
+            )
         prefixes = sorted(prefixes, reverse=True)
         if gid is None:
             if not prefixes: