weechat: fix bad merge

Identified in https://github.com/NixOS/nixpkgs/pull/44102/commits/8887e1f697d9e13ad277ca7d7054bc42c2459548#r239097413.

9504292b accidentally reverted all the
changes that had been made to the weechat wrapper since
8887e1f6.

I removed the wrapper, then wrote it again, but this time taking the
code from the latest version of weechat before the bad merge.