vim modelines for guix repository

[ng0]

Occasionally I use vim on systems to work on guix and other projects. As
far as I see it, there is nothing similar to .dir-locals.el (the per
directory local variables in emacs) for vim.

We should include modelines for every file to reflect what .dir-locals.el
does when you use emacs. It's probably an unpopular opinion, but I think
we should do this to be more open to people who do not use emacs.
Support for vim and emacs covers most editors which are there to
support.