IRCv3: Implement basic CTCP support

This is just the scaffolding for it and doesn't implement any actual commands.

Testing Done:
Sent some `CTCP VERSION`'s to an IRCv3 account. Also manually sent `CTCP VERSION`'s to users and channel via temporary code in when the connection was fully established.

Reviewed at https://reviews.imfreedom.org/r/2854/