feat(neovim): migrate to nixvim with minimal plugins (#260)

Replace bare programs.neovim with programs.nixvim. Only 3 plugins:
neo-tree (file explorer), catppuccin (colorscheme), treesitter.

Add core editor opts matching helix defaults: clipboard, cursorline,
scrolloff, wrap, mouse, undofile, smart search, split direction.
Enable 5 LSP servers: nixd, rust-analyzer, gopls, pyright, bashls.

Signed-off-by: Ryan Yin <xiaoyin_c@qq.com>

flake.nix

@@ -67,6 +67,11 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    nixvim = {
+      url = "github:nix-community/nixvim";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     # https://github.com/catppuccin/nix
     catppuccin = {
       url = "github:catppuccin/nix";