Add Zsh completion script

[He-Ro]

This is the first Zsh completion script I ever wrote, so it might not be perfect.
Mentioning @filipekiss and @muraii, because they seemed interested in a Zsh completion script. Maybe they can test this one out.

This is how it looks for me after typing um <TAB>:
https://ptpb.pw/Apem.png

[sinclairtarget]

Nice! This is great.

I'll have to install zsh in order to make sure this works after getting installed by homebrew. But will try to get around to that soon!

[filipekiss]

Hey @He-Ro! thanks for tagging me! I'll take a look at it and let you know if I find anything

[muraii]

I'll pull this down and test this week. I run with zsh 100% of the time.

[iajrz]

Did anyone get around to testing this?

[sinclairtarget]

Keep meaning to install zsh and try this out. Should have some time this weekend 🤞

[sinclairtarget]

Got around to installing zsh and playing around with this. It works well! Will update the README with information about it once I figure out a good way to make it available once people have installed um