GitHub: add an issue template

[zjeffer]

I'm noticing a lot of issues providing very little information about configuration and Waybar version, so I think it'll improve if you implement an issue template. That way, users will be reminded to provide basic information that will help solve their issue.

A good example of this is Hyprland's template: https://github.com/hyprwm/Hyprland/issues/new/choose. If you choose "Bug report", it will add fields to provide the version, description and steps to reproduce. I think this repo could also benefit from something like this.