coretasks: support `setname` CAP / `SETNAME` command

[dgw]

Description

Implements setname spec. coretasks now updates user's realname when receiving SETNAME event.

Sopel does not yet have a means to change its own realname without reconnecting, but changes to the config currently require a bot restart anyway so that's not a big deal.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches