Onizuka, update neovim config to match update

This merges all the different files into one app.nix using nixvim
instead of the manual home-manager nvim management.

Behavior should be almost unchanged, with the following changes:
- Plugins now have defaults for my own personal sane default experience
- The highlighted line is now 80 instead of 81
- Treesitter plugins may not be integrated correctly

flake.nix

@@ -75,7 +75,6 @@
     #   inputs.lix.follows = "lix";
     # };
 
-
     niri = {
       type = "github";
       owner = "sodiboo";