Build a document editor with undo/redo

• A document holds text and a cursor position; insert(text) inserts at the cursor and delete(count) removes characters before the cursor (backspace); moveCursor(position) is bounded to the text
• Every edit is a command object with execute() and undo(), and is pushed onto an undo stack by an editor that invokes it
• undo() reverses the last command, restoring both text and cursor, and redo() reapplies it; performing a new edit clears the redo stack
• The history has a maximum size (for example 100) and drops the oldest entries first
• Consecutive single-character inserts typed in a row are merged into one undo step (command coalescing), while a cursor move or a pause starts a new group
• undo and redo on an empty stack do nothing and report that, and a text() and cursor() accessor returns the current state

Only after every requirement works.

• Selection with replace and delete selection commands
• Macro (composite) commands that undo as one unit
• Save and restore history to JSON