Improve reusability of kwb.pkgbuild

[mrustl]

@hsonne do you have an idea how the reusability of kwb.pkgbuild could be improved so it can be simple adapted for others. I am thinking about reusing it for https://github.com/DatenHelden

I have in mind to outsource the specific parts (e. g. use_installation()) into the template/config folder.

Ideally changes would need to be done only there without need to overwrite the function defaults as these would be based on the template/config.

At least I started with the template approach (also used by the usethis package) but I have no idea how to proceed with others now