Make most WeeChat identifiers case sensitive

[flashcode]

Feature description

Many WeeChat identifiers are currently case insensitive, for example: configuration files/sections/options, command names and parameters, completion, etc.

Most of them should be case sensitive so that for example an alias /AWAY would be different from the /away command, or a parameter in upper case would be different as well, eg: /test -o is different from /test -O.

Specification: https://specs.weechat.org/specs/2023-001-case-sensitive-identifiers.html

[weechatter]

I like the idea of having two different command like, /AWAY and /away. Autocompletion should be more strict, as described in the specs.