use OMZ in environment variables instead of ZSH

[rwmitchell]

I acknowledge this would be a major effort and is therefore very unlikely to ever happen, but...

ZSH points to the location for oh-my-zsh, as such, it would be better named OMZ
There are plenty of other environment variables (ie: ZSH_THEME) that should also be renamed (OMZ_THEME).

The advantage is to make it clearer to newcomers where the separation between ZSH and OMZ is.
It also makes it more obvious which a variable is referring. ZSH_VERSION is actually the ZSH version (as it should be), but that isn't clear to someone just getting started.

There must be a way to phase it in, allowing both for a short time while alerting old users to make the switch.