[Feature Request] Retrieve user for the first user arg from the message reference

[segvyfault]

Currently, if command function defined like this:

fn command(user1: User, user2: User)

Poise will only call this function if you do this:

!command @user1 @user2

But I want it to also accept this type of ping:

- replying to @user1 (with mentioning on !!, otherwise it should be ignored)
!command @user2

I tried forking and trying to implement it on my own, but the code of prefix argument parsing is too complex and non-flexible, so I'll leave it for you, the maintainers.
I did implement that before in just serenity, and instead of parsing userids from the message content, I used message.mentioned_users or whatever its called. If the mention on referenced message was enabled, first user will always be the author of the replied message