Add cornelis plugin definition for NixVim

[flgrubm]

NixVim is a Neovim distribution that allows configuration of Neovim via the nix language. Maybe this is out of scope, but I thought it might not be, as cornelis also maintains a nix flake already. Plugin definitions for other proof assistants like Coq (coq-lsp) and Lean (lean-nvim) already exist.

I already made a plugin request for cornelis on the NixVim Github repository quite a while ago: nix-community/nixvim#2280