Add way to mention slash commands

[onerandomusername]

Summary

Slash commands can be mentioned, should have a mention attribute on slash command instances.

What is the feature request for?

The core library

The Problem

slash mentions have to be manually constructed

The Ideal Solution

a property which makes a slash command mention, like user, channel, and role mentions

The Current Solution

No response

Additional Context

No response

[shiftinv]

https://github.com/discord/discord-api-docs/pull/5186