`u`/`<C-r>` for undo/redo don’t work in the interactive playground

[chris-morgan]

To reproduce: Steps to reproduce the behavior:

1. Help → Interactive Playground.
2. Select one of the editable areas.
3. Modify the text in some way.
4. Press u to undo the last change.

Expected behavior: It should undo the change.

Actual behaviour: “Already at oldest change” status text; nothing happens. (Much the same on Ctrl+R for redo when there are redoable changes.)

Note that the Visual Studio Code’s native Ctrl+Z and Ctrl+Shift+Z do work.

Environment:

• Extension (VsCodeVim) version: 1.14.2
• VSCode version: 1.45.0
• OS: Windows 10.0.19041

[catskul]

This appears to be true of the editor in general. I don't recall this being a problem until recently.