Consider our own undo/redo stack for the editor

Currently certain operations do not undo correctly, or clear the undo history.

Most recent report: https://forums.ankiweb.net/t/in-editor-keyboard-undo-redo-breaks-with-formatted-text-and-loses-history-when-field-unfocused-and-refocused/47688

The issue names no files, tests, or entry points. Start by reviewing the linked forum report and reproducing the formatted-text and focus-change cases; done means undo/redo preserves history and correctly reverses the affected editor operations.

Written by the indexing model from the issue text.