Replied user mention is not included in Message.MentionedUsers

Summary

When a message_reference is received with the original user being mentioned, the mention does not appear in Message.MentionedUsers.

Details

.NET version: net5.0
DSharpPlus version: 4.0.0-rc1

Steps to reproduce

1. Send a message that includes a reply, with the "ping the original author" button enabled.
2. Observe the Message.MentionedUsers list being empty.

[Kiritsu]

Hello, this is not implemented yet, but there's a PR for it! See: #704

Ah, thank you