Add virtualenv support to the ZSH prompt

[duganchen]

This adds support for Python's Virtual environments to the ZSH prompt.

So far, I've updated only the code (it works). I want to see if there's interest in merging this before I update the comments and documentation.

[arcticicestudio]

Hi @duganchen 👋, thanks for your contribution 👍
Even though I appreciate every help I don't want to merge this one. The reason is that these are my personal .files and I try to avoid anything related to scripting languages like Python.
I also like to keep my prompt as simple as possible without introducing any "clutter" or fill it up with tons of features that have only little benefit or are never used. I know many users got used to shell/prompt bootstrap projects like "Oh My ZSH" or spaceship that come with many features likes this, but keeping it simple is a great way to keep focus on the important parts.

Anyway, this is open source and you can simply use it for your own setup. One way to improve this PR is to use ZSH's condition syntax instead of an empty VENV variable:

(?..${VENV})

Thanks again for your effort to create this PR. Rejecting a PR is one of the harder parts as a project maintainer so I hope you understand my points.