Add support for bot mode spec

[examknow]

Closes #210

Link to spec for reference: https://ircv3.net/specs/extensions/bot-mode

[edk0]

I don't want draft caps. Independently of that I'm far from convinced that the bot mode is a good feature.

[jesopo]

I'm also unconvinced this is the correct solution. I'd be a lot more interested in implementing the WIP metadata spec in anticipation of using that to denote bottiness

[jesopo]

@TheDaemoness how do we feel about this these days

[TheDaemoness]

(Pinging me because I've been a proponent for bot-mode on Libera staff.)

Seeing as metadata fell apart and Weechat now supports bot mode, I am generally in favor of implementing it, with perhaps the caveat that it should be possible for services to set this mode on users to facilitate things like bot accounts.

Also, this PR appears to be outdated: it calls for a draft/bot capability that is not present in the final spec.