I added undo and redo!

Basically, every mutative keystroke (e.g. space or backspace) makes a snapshot of the current lines and editor position and adds it to the history slice. When you press ctrl+z, it decrements historyIndex and replaces the current lines and cursor position with history[historyIndex]. ctrl+y does the same thing but the opposite.

Oh and I also added word-skipping cursor movement. If you press ctrl+left or ctrl+right, the cursor will move in that direction until it reaches a delimiter (e.g. a space or a punctuation). It took embarrassingly long to get it to work perfectly. There’s a bunch of little edge case behaviors for cursor caret movement that are really subtle and kinda unintuitive, but which you’re so used to from writing text on computers that you can instantly spot if it’s not quite correct.

I think that next I’ll start working on the README.