modules/diagnostic: rename diagnostics -> diagnostic.config

Currently we represent `vim.diagnostic.config()` as a top-level
`diagnostics` option. This means we have no clear namespace for
(e.g.) `vim.diagnostic.<action>` keymap functions.

modules/default.nix

@@ -10,7 +10,7 @@
     ./colorscheme.nix
     ./commands.nix
     ./dependencies.nix
-    ./diagnostics.nix
+    ./diagnostic.nix
     ./doc.nix
     ./editorconfig.nix
     ./files.nix

modules/diagnostic.nix

@@ -0,0 +1,32 @@
+{
+  lib,
+  config,
+  ...
+}:
+let
+  cfg = config.diagnostic;
+in
+{
+  options.diagnostic = {
+    config = lib.mkOption {
+      type = with lib.types; attrsOf anything;
+      default = { };
+      description = "The configuration diagnostic options, provided to `vim.diagnostic.config`.";
+      example = {
+        virtual_text = false;
+        virtual_lines.current_line = true;
+      };
+    };
+  };
+
+  imports = [
+    # TODO: Added 2025-04-26; remove after 25.05
+    (lib.mkRenamedOptionModule [ "diagnostics" ] [ "diagnostic" "config" ])
+  ];
+
+  config = {
+    extraConfigLuaPre = lib.mkIf (cfg.config != { }) ''
+      vim.diagnostic.config(${lib.nixvim.toLuaObject cfg.config})
+    '';
+  };
+}

modules/diagnostics.nix

@@ -1,25 +0,0 @@
-{
-  lib,
-  helpers,
-  config,
-  ...
-}:
-{
-  options = {
-    diagnostics = lib.mkOption {
-      type = with lib.types; attrsOf anything;
-      default = { };
-      description = "The configuration diagnostic options, provided to `vim.diagnostic.config`.";
-      example = {
-        virtual_text = false;
-        virtual_lines.current_line = true;
-      };
-    };
-  };
-
-  config = {
-    extraConfigLuaPre = lib.mkIf (config.diagnostics != { }) ''
-      vim.diagnostic.config(${lib.nixvim.toLuaObject config.diagnostics})
-    '';
-  };
-}