[Enhancement] Format message mentions into human-readable version

Created by: Evengard

I have a bot which retransmits and shows some messages on my website. The problem is that the mentions are not really human-readable - it's just a bunch of cryptic tags with snowflake identifiers, which are useless for a simple user. For now I just transform it simply and dirty with Formatter.Strip(), but it would be nice to have a method which would output the message into something similar to what is shown in the Discord client itself. Aka Some message with mention <@!1111111111111> would be transformed into Some message with mention @GuildUsername. I probably could do it myself, but I think other people could find this useful too.