shell-scripts: Use [[ instead of [ where possible

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.

.config/zsh/plugins/alias.zsh

@@ -59,7 +59,7 @@
 		&& sudo apt upgrade -y \
 		&& sudo apt autoremove -y
 
-		[ ! -e /var/run/reboot-required ] \
+		[[ ! -e /var/run/reboot-required ]] \
 		|| printf "\n\nSystem restart required.\n"
 	'
 	alias pdf2t{e,}xt='pdftotext'