feat(editor): update undo-tree to 0.8

Override the undo-tree package within straight.el to have it grab the
package directly from the official Git repo on GitLab.

modules/editor/siren-undo-tree.el

@@ -24,7 +24,10 @@
   :custom
   (undo-tree-history-directory-alist
    `((".*" . ,(expand-file-name "undo-tree-history" siren-cache-dir))))
-  (undo-tree-auto-save-history nil) ;; use undohist package instead
+  ;; Use undohist package to persist history to disk, it seems more reliable
+  ;; than undo-tree's auto-save feature which randomly fails to restore history
+  ;; for no obvious reason.
+  (undo-tree-auto-save-history nil)
 
   :config
   ;; Unbind keys that I don't use.