Improve document debugging view

[cloverich]

When changing or customizing Slate components and / or document save / load, there are various transform steps that happen to the Slate DOM or saved content. I frequently need to see various intermediate transform steps to properly debug. Ideally there would be a debug view that:

• Could be easily toggled on / off
• Could track history
• Could be implemented fully separate from the existing editor, i.e. the existing editor's content would flow through the debug view, but it would otherwise be unaware off its existence (code fully separated)

I've implemented rudimentary versions of it; here is a rough sketch