Add the ability to get user by name/discrim

[SylveonDeko]

I feel like this will be useful in cases where you aren't able to get the ID of a member, for example a reason in audit logs that just says the username, for example in my bot it says when banning: Sylveon#0001 | reason here

[quinchs]

This isn't possible if the user isn't in a guild.

If they are in a guild you can use SearchUsersAsync("user#1234").