[V3 core.commands] decorator inheritence fix (#1786)

* decorator inheritence

* black format

redbot/core/commands/commands.py

@@ -48,6 +48,32 @@ class Command(commands.Command):
         # We don't want our help property to be overwritten, namely by super()
         pass
 
+    def command(self, cls=None, *args, **kwargs):
+        """A shortcut decorator that invokes :func:`.command` and adds it to
+        the internal command list via :meth:`~.GroupMixin.add_command`.
+        """
+        cls = cls or self.__class__
+
+        def decorator(func):
+            result = command(*args, **kwargs)(func)
+            self.add_command(result)
+            return result
+
+        return decorator
+
+    def group(self, cls=None, *args, **kwargs):
+        """A shortcut decorator that invokes :func:`.group` and adds it to
+        the internal command list via :meth:`~.GroupMixin.add_command`.
+        """
+        cls = None or Group
+
+        def decorator(func):
+            result = group(*args, **kwargs)(func)
+            self.add_command(result)
+            return result
+
+        return decorator
+
 
 class Group(Command, commands.Group):
     """Group command class for Red.