tweak(vc): smerge-next -> smerge-vc-next-conflict

This is provided by `smerge-mode` in Emacs 27, and functions exactly
like `smerge-next`, except that if there are no more conflicts in the
current file, it will go to the next file with conflicts.

Arguably this could be a different key binding. However, I find that it
is almost strictly superior to `smerge-next`: I can't think of a case
where I *wouldn't* immediately want to go to the next file with
conflicts, and even if you do by accident you can just pop back to the
preceding buffer. So I think it warrants being the default.

Close: doomemacs#5431

modules/emacs/vc/autoload/hydra.el

@@ -20,7 +20,7 @@
 "
     ("g" (progn (goto-char (point-min)) (smerge-next)))
     ("G" (progn (goto-char (point-max)) (smerge-prev)))
-    ("C-j" smerge-next)
+    ("C-j" smerge-vc-next-conflict)
     ("C-k" smerge-prev)
     ("j" next-line)
     ("k" previous-line)