Replace undo-tree with undo-fu + undo-fu-session

Despite the recent patches to fix undo history corruption, its history
would still get unpredictably truncated due to non-linear memory
expansion.

Relevant to #2339

modules/ui/ophints/config.el

@@ -31,7 +31,6 @@
   :unless (featurep! :editor evil)
   :after-call pre-command-hook
   :config
-  (volatile-highlights-mode)
-  (after! undo-tree
-    (vhl/define-extension 'undo-tree 'undo-tree-yank 'undo-tree-move)
-    (vhl/install-extension 'undo-tree)))
+  (after! undo-fu
+    (vhl/define-extension 'undo-fu 'undo-fu-only-undo 'undo-fu-only-redo)
+    (vhl/install-extension 'undo-fu)))