plugins/lsp: add nixd language server

tests/test-sources/plugins/lsp/nixd.nix

@@ -0,0 +1,35 @@
+{pkgs}: {
+  example = {
+    plugins.lsp = {
+      enable = true;
+
+      servers.nixd = {
+        # TODO nixd is currently broken on Darwin
+        # https://github.com/nix-community/nixd/issues/107
+        # Thus, this test is currently disabled.
+        enable = !pkgs.stdenv.isDarwin;
+
+        settings = {
+          eval = {
+            target = {
+              args = ["foo" "bar"];
+              installable = "";
+            };
+            depth = 0;
+            workers = 3;
+          };
+          formatting = {
+            command = "nixpkgs-fmt";
+          };
+          options = {
+            enable = true;
+            target = {
+              args = ["yes" "no" "maybe"];
+              installable = "";
+            };
+          };
+        };
+      };
+    };
+  };
+}