add undo and history

[ryanatkn]

The editor currently lacks any form of undo. It's a tricky thing to get right, because the desired UX differs across contexts, and it can be hard in general to implement.

Luckily we're on a good track using JSON backing all of the classes with Svelte's $state.snapshot. Implementing some basic undo and history, using graphics editors as a guide, will help inform some architectural decisions.