home: git: extract 'delta' configuration

I want to be able to re-use it between different source control systems
(e.g: `jj`).

As a first step, extract it to a proper module so that I can have it
live in a single space.

modules/home/default.nix

@@ -8,6 +8,7 @@
     ./bluetooth
     ./calibre
     ./comma
+    ./delta
     ./dircolors
     ./direnv
     ./discord

modules/home/delta/default.nix

@@ -0,0 +1,61 @@
+{ config, pkgs, lib, ... }:
+let
+  cfg = config.my.home.delta;
+in
+{
+  options.my.home.delta = with lib; {
+    enable = my.mkDisableOption "delta configuration";
+
+    package = mkPackageOption pkgs "delta" { };
+
+    git = {
+      enable = my.mkDisableOption "git integration";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        # For its configuration
+        assertion = cfg.enable -> cfg.git.enable;
+        message = ''
+          `config.my.home.delta` must enable `config.my.home.delta.git` to be
+          properly configured.
+        '';
+      }
+    ];
+
+    home.packages = [ cfg.package ];
+
+    programs.git = lib.mkIf cfg.git.enable {
+      delta = {
+        enable = true;
+        inherit (cfg) package;
+
+        options = {
+          features = "diff-highlight decorations";
+
+          # Less jarring style for `diff-highlight` emulation
+          diff-highlight = {
+            minus-style = "red";
+            minus-non-emph-style = "red";
+            minus-emph-style = "bold red 52";
+
+            plus-style = "green";
+            plus-non-emph-style = "green";
+            plus-emph-style = "bold green 22";
+
+            whitespace-error-style = "reverse red";
+          };
+
+          # Personal preference for easier reading
+          decorations = {
+            commit-style = "raw"; # Do not recolor meta information
+            keep-plus-minus-markers = true;
+            paging = "always";
+          };
+        };
+      };
+    };
+  };
+}

modules/home/git/default.nix

@@ -42,34 +42,6 @@ in
 
     lfs.enable = true;
 
-    delta = {
-      enable = true;
-
-      options = {
-        features = "diff-highlight decorations";
-
-        # Less jarring style for `diff-highlight` emulation
-        diff-highlight = {
-          minus-style = "red";
-          minus-non-emph-style = "red";
-          minus-emph-style = "bold red 52";
-
-          plus-style = "green";
-          plus-non-emph-style = "green";
-          plus-emph-style = "bold green 22";
-
-          whitespace-error-style = "reverse red";
-        };
-
-        # Personal preference for easier reading
-        decorations = {
-          commit-style = "raw"; # Do not recolor meta information
-          keep-plus-minus-markers = true;
-          paging = "always";
-        };
-      };
-    };
-
     # There's more
     extraConfig = {
       # Makes it a bit more readable