lib/extend-lib.nix returns a nixpkg's lib extended with our own
helpers.
This is exposed as `helpers.extendedLib`, but when evaluating our
modules it should be assigned to `specialArgs.lib`.
Outside of our modules you must still access our helpers via
`config.lib.nixvim` or `config.lib.nixvim.extendedLib`.
Within helpers' sub-sections, `lib` is the extended lib.
