End-users have ran into issues before when attempting to re-use a `lib`
from elsewhere in nixvim's configuration.
If a `lib` without the `nixvim` extension is used, this assertion will
inform the user that this isn't supported.
In the future we can also provide better ways for end-users to overlay
their own lib with nixvim's extensions, but this is not yet implemented.
Co-authored-by: Gaétan Lepage <33058747+GaetanLepage@users.noreply.github.com>