Complete IRC numerics module and move to pkg/irc/ (refs #52) (#71)

This PR addresses [issue #52](#52):

- **Adds all missing numeric reply codes** from RFC 1459 and RFC 2812, making the module spec-complete
- **Moves the package** from `internal/irc/` to `pkg/irc/` to indicate external usefulness
- **Updates all imports** throughout the codebase

### Added numerics

- Trace replies (200-209)
- Stats replies (211-219, 242-243)
- Service replies (234-235)
- Admin replies (256-259)
- Trace log/end, try again (261-263)
- WHOWAS (314, 369)
- List start (321), unique ops (325)
- Invite/except lists (346-349)
- Version (351), links (364-365)
- Info (371, 374)
- Oper/rehash/service (381-383)
- Time/users (391-395)
- All missing error codes (406-415, 422-424, 436-437, 443-446, 463-467, 472, 476-478, 481, 483-485, 491, 501-502)

refs #52

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #71
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

pkg/irc/commands.go

@@ -0,0 +1,21 @@
+package irc
+
+// IRC command names (RFC 1459 / RFC 2812).
+const (
+	CmdJoin    = "JOIN"
+	CmdList    = "LIST"
+	CmdLusers  = "LUSERS"
+	CmdMode    = "MODE"
+	CmdMotd    = "MOTD"
+	CmdNames   = "NAMES"
+	CmdNick    = "NICK"
+	CmdNotice  = "NOTICE"
+	CmdPart    = "PART"
+	CmdPing    = "PING"
+	CmdPong    = "PONG"
+	CmdPrivmsg = "PRIVMSG"
+	CmdQuit    = "QUIT"
+	CmdTopic   = "TOPIC"
+	CmdWho     = "WHO"
+	CmdWhois   = "WHOIS"
+)