[Feature] Channel mode to block IRC formatting codes (+c) #122

Summary

Add a channel mode that lets channel operators keep IRC formatting codes out of a channel's messages. Many servers offer this as +c.

Background

IRC formatting is done with C0 control characters in message text:

• \x02 bold
• \x03 color, for example \x0304,01
• \x04 hex color
• \x0F reset
• \x11 monospace
• \x16 reverse
• \x1D italic
• \x1E strikethrough
• \x1F underline

jircd delivers these unchanged today, and channels have no way to restrict them. The core channel modes are b e I i m n p s o v l k t. There is no formatting mode, and channel modes can also come from extensions (ExtensionRegistry.recognizedChannelModes).

Proposal

• A boolean channel mode +c, settable by channel operators. While it is set, a PRIVMSG or NOTICE to the channel that contains formatting codes is rejected with 404 ERR_CANNOTSENDTOCHAN, and it is not delivered to anyone.
• It is advertised in CHANMODES (type D) and in 004, like the other boolean modes.
• It is delivered as a module through the existing extension mechanism, following the capability-plugin intent, so it stays out of core.

Open questions (settle in the spec)

• Block or strip? Some servers block with +c and offer a separate mode, often +S, that strips the codes and delivers the cleaned text. Do we want one of these or both?
• Which codes? Colors only (\x03, \x04), or all formatting codes?
• CTCP: \x01 framing, such as ACTION, must keep working. Does a /me that contains colors count?
• Other text: do TOPIC, PART and KICK reasons in a +c channel follow the same rule, or only messages?
• Exemptions: are operators and voiced members exempt?

Out of scope

Formatting codes in identifiers. #121 rejects control characters in USER idents.