feat(vc): integrate smerge-mode

Activates smerge-mode if the file contains merge conflict markers. Also
turns binds the localleader to `smerge-mode-map` when it's active.

Also removes an unused leader binding (which will never be set because
:ui hydra was removed in b08c2c7).

Amend: b08c2c745f
Close: #5954

modules/emacs/vc/config.el

@@ -45,6 +45,20 @@
   (set-evil-initial-state! 'vc-dir-mode 'emacs))
 
 
+(use-package! smerge-mode
+  :defer t
+  :init
+  (add-hook! 'find-file-hook
+    (defun +vc-init-smerge-mode-h ()
+      (unless (bound-and-true-p smerge-mode)
+        (save-excursion
+          (goto-char (point-min))
+          (when (re-search-forward "^<<<<<<< " nil t)
+            (smerge-mode 1))))))
+  :config
+  (define-key smerge-mode-map (kbd doom-localleader-key) smerge-basic-map))
+
+
 (after! git-timemachine
   ;; Sometimes I forget `git-timemachine' is enabled in a buffer, so instead of
   ;; showing revision details in the minibuffer, show them in