feat(version-control/magit): add magit-delta package

Disabled by default, as the syntax highlighting can make code harder to
read in the diff. Hence disabled by default, and easy to toggle on if I
want to try it.

modules/version-control/siren-magit.el

@@ -66,5 +66,29 @@
   (transient-values-file
    (expand-file-name "transient/values.el" siren-cache-dir)))
 
+(use-package magit-delta
+  :hook
+  (magit-mode . siren-setup-magit-delta)
+
+  :custom
+  (magit-delta-default-dark-theme "OneHalfDark")
+  (magit-delta-default-light-theme "OneHalfLight")
+  (siren-magit-delta-auto-enable nil)
+
+  :init
+  (defgroup siren-magit-delta nil
+    "Siren specific options for magit-delta."
+    :group 'magit-delta)
+
+  (defcustom siren-magit-delta-auto-enable t
+    "Automatically enable magit-delta-mode when delta executable is available."
+    :type 'boolean
+    :group 'siren-magit-delta)
+
+  (defun siren-setup-magit-delta ()
+    (let ((delta (executable-find "delta")))
+      (when delta
+        (magit-delta-mode (if siren-magit-delta-auto-enable +1 -1))))))
+
 (provide 'siren-magit)
 ;;; siren-magit.el ends here