Add support for IRCv3 Client Capability Negotiation

[hloeung]

This is for https://github.com/42wim/matterircd/issues/476

Testing using irssi:

INFO[2022-10-12T19:43:04+11:00] New connection: 127.0.0.1:54024               module=matterircd
DEBU[2022-10-12T19:43:04+11:00] using paste buffer timeout: 1000              module=matterircd
DEBU[2022-10-12T19:43:04+11:00] <- CAP LS 302                                 module=matterircd
DEBU[2022-10-12T19:43:04+11:00] <- JOIN :                                     module=matterircd
DEBU[2022-10-12T19:43:04+11:00] -> :matterircd CAP * LS :                     module=matterircd
DEBU[2022-10-12T19:43:04+11:00] <- CAP END                                    module=matterircd
DEBU[2022-10-12T19:43:04+11:00] <- NICK hloeung                               module=matterircd

Testing with netcat:

$ nc -C -v 127.0.0.1 6697
Connection to 127.0.0.1 6697 port [tcp/ircs-u] succeeded!
CAP LS 302
:matterircd CAP * LS :
CAP DDDD
:matterircd 410 * DDDD :Invalid or unsupported CAP command

[42wim]

Thanks for your work on this!

[hloeung]

Thanks for reviewing!