Replace all occurrences of [ with [[ in bash and zsh scripts and
configs.
Performance wise it makes sense to use the builtin instead of calling an
external command also when from a functionality stand point `test` would
suffice.
Instead of hardcoding the paths of all the scripts, use env variables
that are set in launch.sh. This way the config also works with
XDG_CONFIG_HOME not set to ~/.config.
Sadly polybar only accepts references that stand on their own.
(So something like ${env:XDG_CONFIG_HOME:~/.config}/polybar/<script>
does not work.) Hence the mess with the associative array and the
variables for every module.
