Fancier issue templates, to match Nixpkgs

[athyfr]

Description

Nixpkgs's issue templates use all of GitHub's new features for guided issues, which make the process much more informational and formal than just a blank slate.

Since issues for home-manager might be quite similar to ones for Nixpkgs, you may as well template it from Nixpkgs issues.

This might make things more manageable. (Reduce duplicate issues, make issues more thorough, etc.)

Thanks for the awesome module!