IRCv3 replies and reactions

[4e554c4c]

There are two IRC features driven by client-only tags that are related: replies and reactions. These features allow for an experience more similar to a modern chat application. It would be nice if halloy could add support for these features after msgid support is added.

[reesericci]

Would it make sense to split this into two issues, threads/replies and reactions?

[4e554c4c]

the main issue is that reactions kind of depend on replies, since a reaction is a special kind of reply

but you don't need to support all of replies to support reactions and vice versa. confusingly, the reference direction in code is usually opposite for replies and reactions as well (which is why in goguma we have two different msgid lookup tables, one for replies and one for reacts)