feat: add IRCv3 capabilities, WHO/WHOX, MONITOR and message splitting

[jeromeludmann]

Summary

Add comprehensive IRCv3 support with 14 new plugins, enhanced core protocol handling, and full test coverage.

Core

• Add ACCOUNT, CHGHOST, SETNAME, TAGMSG, MONITOR commands and numerics (730-734) to protocol
• Add IRCv3 tag value escaping/unescaping in parser
• Add outgoing tag support in send() via overload (send(tags, command, ...params))

IRCv3 capability plugins

• cap: Track ACK/NAK/NEW/DEL, batch CAP REQ into single message, auto-request cap-notify
• account-notify: Emit account event on user login/logout
• account-tag: Expose getAccount() util from message tags
• away-notify: Emit away_notify event on away status change
• chghost: Emit chghost event on host/ident change
• echo-message: Intercept self-messages via emit hook, emit echo:privmsg / echo:notice
• extended-join: Emit extended_join with account and realname
• invite-notify: Broadcast invites to channel members
• message-tags: Send/receive TAGMSG with client tags
• server-time: Expose getServerTime() util
• setname: Emit setname event + setname() command

Other plugins

• who: WHO/WHOX with buffered replies and token matching
• monitor: Full MONITOR support (add/remove/list/clear/status + online/offline events)
• message-split: Auto-split long PRIVMSG/NOTICE at word boundaries, byte-aware for multi-byte
• isupport: Generic client.state.isupport map for all ISUPPORT params

Fixes

• Fix SASL authentication with batched CAP REQ
• Fix echo-message suppressing normal privmsg/notice for self-messages
• Remove circular dependency: registration no longer depends on privmsg

Docs

• Document all new events, commands, and options in API.md
• Add IRCv3 bullet to README

Test plan

• 345 unit tests (84 new), all passing on Deno and Node.js
• 21 E2E tests against real Ergo IRC server (12 new), all passing
• E2E covers: server-time, away-notify, echo-message, setname, WHO, MONITOR, message-split, cap tracking, invite-notify, chghost, TAGMSG, extended-join, account-tag, SASL PLAIN, SASL EXTERNAL