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/90-external-plugins/70-direnv

@@ -1 +1 @@
-! command -v direnv &>/dev/null || eval "$(direnv hook zsh)"
+! (( $+commands[direnv] )) || eval "$(direnv hook zsh)"