zsh: Use dictionary for command existence check

Use the `commands` dictionary from zsh instead of always calling
`command`.

Regex used:
s/command -v \([^ ]*\) &>\/dev\/null/(( $+commands[\1] ))/g

.config/zsh/zshrc.d/40-functions.zsh

@@ -48,9 +48,9 @@ conf() {
 	local CONF_EDITOR
 	if [[ -n "$EDITOR" ]]; then
 		CONF_EDITOR="$EDITOR"
-	elif command -v vim &>/dev/null; then
+	elif (( $+commands[vim] )); then
 		CONF_EDITOR=vim
-	elif command -v nano &>/dev/null; then
+	elif (( $+commands[nano] )); then
 		CONF_EDITOR=nano
 	else
 		CONF_EDITOR=cat