Add event(once=True) to Cog.listener

[DTheIcyDragon]

Summary

Adding the once variable to Cog listeners

What is the feature request for?

discord.ext.commands

The Problem

The @commands.Cog.listen() does not have the once=True variable to set.

The Ideal Solution

Add the once variable to the decorator

The Current Solution

No response

Additional Context

This is tested on the latest github commit

[Dorukyum]

The _once attribute is used for the normal event listeners but it can't be added to cog listeners because they're methods of a class. Once-listeners must be stored in some other way for this to be added.