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/autoload/ls

@@ -20,9 +20,9 @@ builtin local -a dirs files
 # All other arguments are kept.
 for arg in "$@"; do
 	shift
-	if [ -d "$arg" ]; then
+	if [[ -d "$arg" ]]; then
 		dirs+="$arg"
-	elif [ -e "$arg" ]; then
+	elif [[ -e "$arg" ]]; then
 		files+="$arg"
 	else
 		set -- "$@" "$arg"
@@ -36,7 +36,7 @@ fi
 
 # Do not separate files and directories if -d flag was specified.
 while getopts d flag 2>/dev/null; do
-	if [ "$flag" = "d" ]; then
+	if [[ "$flag" = "d" ]]; then
 		command ls "$@" -- "${files[@]}" "${dirs[@]}"
 		return
 	fi