Support all allowed channel prefixes

RFC 2812 specifies in §1.3 that “Channels names are strings (beginning
with a '&', '#', '+' or '!' character) of length up to fifty (50)
characters.”

We previously only allowed the first two prefix characters, or any other
one if the server overrides the list with ISUPPORT.

src/irc/irc_client.cpp

@@ -155,7 +155,7 @@ IrcClient::IrcClient(std::shared_ptr<Poller>& poller, std::string hostname,
   bridge(bridge),
   welcomed(false),
   chanmodes({"", "", "", ""}),
-  chantypes({'#', '&'}),
+  chantypes({'&', '#', '+', '!'}),
   tokens_bucket(this->get_throttle_limit(), 1s, [this]() {
     if (message_queue.empty())
       return true;