Data: Build block playground (REPL)

[dmsnell]

While work on the UI and block registration process progresses it would be handy to have an area where we could continue to develop the data model and operations without worrying about all of those other concerns. This would be a place to test operations like "insert block" or "select this string and turn it into a list block."

I'm imagining two big panes across the top two-thirds of the window, splitting it in half, where the left pane contains the document model in JS object form before a requested operation and the right pane shows the resulting data structure from applying that operation. The lower third of the window contains the REPL where we can issue code by calling existing functions in the project.

This should be particularly useful for us as we try to understand what structure we need for parsed/lexed HTML Elements in block content and how we want to modify them.

[dmsnell]

Too old too sad.