Added shell completions for zsh

[LexMalta]

Summary

Simple script to get shell/bash completions for zsh working.

I honestly don't know much about shell completions or how they work, but a quick bit of googling found: eddiezane/lunchy#59 (comment) .I tried that locally and it worked for me - figured it would be worth sharing upstream.

Are there any docs that need to be updated?

[jthegedus]

Hi mate, thanks for contributing.

Unfortunately, this is code we used to have in our repo but have since removed. This implementation of using the Bash completions was how we originally supported ZSH users for a few years, but we now have native ZSH completions.

Issues covering that reversal are here #674 #692

The documentation site https://asdf-vm.com/#/core-manage-asdf-vm covers how to setup the Shell completions for ZSH vs Bash etc.

If you find yourself itching to contribute more we have plenty of improvements to make, thanks!