vim: Add U to undo last line (#33571)

Closes #14760

Still TODO:

* Vim actually undoes *many* changes if they're all on the same line.

Release Notes:

- vim: Add `U` to return to the last changed line and undo

assets/keymaps/vim.json

@@ -364,6 +364,7 @@
       "p": "vim::Paste",
       "shift-p": ["vim::Paste", { "before": true }],
       "u": "vim::Undo",
+      "shift-u": "vim::UndoLastLine",
       "r": "vim::PushReplace",
       "s": "vim::Substitute",
       "shift-s": "vim::SubstituteLine",