Add undo/redo

[amir9480]

There is one limitation that I couldn't fix. after undo/redo, values in the inspect tab are not updating.

[jordanisip]

Fantastic work, @amir9480. Appreciate the contribution. I'll leave the actual code review to @cohitre but a couple quick notes from my end:

• I believe we can use the 'metaKey' keyboard event so it works for the different modifiers on mac vs pc. (Edit: it sounds like metaKey actually maps to the windows key on PC so maybe we'd need (event.ctrlKey || event.metaKey) to support cross-platform keyboard shortcuts?
• It appears the first block that gets added doesn't activate the undo. I would have expected to undo back to the empty state in this case: