Bundle `bash-preexec` with Resh shell files

[curusarn]

Somehow try to check if bash-preexec is loaded and only source it if it's not.
This way we can bundle bash-preexec into resh shell scripts instead of sourcing it separately.
Once we bundle bash-preexec then resh won't depend on .bash-preexec.sh file being present in user home.
This helps with #73