It turns out the only thing we need to save in the appState is the background color. All the rest is transient data.
I added `"type": "excalidraw"` at the beginning to explain where it was.
I removed `"source": "http://localhost:3000/"`. I don't think we want to leak on which webpage it was saved from.
I removed `isSelected` from the json
I added indentation so it's easier to read the content. I'm not 100% sure on this one, but I figure filesize doesn't matter too much those days. And if we want to shrink it, there are more effective ways than json.