Add Zig support

[Polairr]

This will grab the major version of Zig from apt. I've tested this on an Ubuntu 18.04 EC2 instance, and it seems to work without any problems.

[dan-stowell]

Good afternoon @Polairr ! You've probably noticed by now that we've been slow to merge community PRs on polygott. We've decided to focus on giving Repl.it a stable foundation instead of adding features and updating languages. Once we and our community feel comfortable with Repl.it's overall stability, we'll give this change another look. Thank you for contributing!