Add nvim-lazy submodule and add plugin configuration.

pkgsets/nvim/lsp/nix-nixd.nix

@@ -0,0 +1,42 @@
+{
+  lib,
+  config,
+  pkgs,
+  ...
+}:
+
+lib.optionalString (lib.elem "nvim::lsp::nix-nixd" config.machine.pkgs) # lua
+  ''
+    lspconfig.nixd.setup({
+      cmd = { "${pkgs.nixd}/bin/nixd" },
+      settings = {
+        nixd = {
+        -- on_init = function(client)
+        --     local path = client.workspace_folders[1].name
+
+        --     if path == '/path/to/project1' then
+        --       client.config.settings["rust-analyzer"].checkOnSave.overrideCommand = { "cargo", "check" }
+        --       client.notify("workspace/didChangeConfiguration", { settings = client.config.settings })
+        --     end
+
+        --     return true
+        --   end,
+          nixpkgs = {
+            expr = 'import (builtins.getFlake ("git+file:///etc/nixos")).inputs.nixpkgs {}',
+          },
+          formatting = {
+            command = { "${pkgs.nixfmt-rfc-style}/bin/nixfmt" },
+          },
+          options = {
+            nixos = {
+              expr = '(builtins.getFlake ("git+file:///etc/nixos")).outputs.nixosConfigurations.Lilim.options',
+            },
+            -- TODO: Find a good general expression for this.
+            -- home_manager = {
+            --   expr = '(builtins.getFlake ("git+file://" + toString ./.)).homeConfigurations."ruixi@k-on".options',
+            -- },
+          },
+        },
+      },
+    })
+  ''