Reflect bug fix in Zsh 5.8

The subst-async method has always included workarounds for a bug in Zsh itself which has been fixed in Zsh v5.8. This update makes changes to agkozak-zsh-prompt that eliminate unnecessary forking if the shell version is sufficiently recent and subst-async is in use. The majority of users default to the usr1 async method, however, so they should not be affected.