Ensure that we're using the builtin exec

[jeebak]

I have a wrapper shell function for exec() in my environment which started to behave unexpectedly after I updated zsh-autosuggestions. I'm using the unset ZSH_AUTOSUGGEST_USE_ASYNC suggestion from the README as a workaround for now, but it'd be nice to enable async. This PR added the builtin precommand modifier to all occurances to exec in the code to ensure we use the builtin exec command "rather than a shell function or external command".