Experimental Editable state

[ellatrix]

This is a WIP. See #771.

[dmsnell]

As discussed in person, I feel like we will be able to make much better judgments about how the data structure will affect our coding and the editor if we have a playground in which to test it. We need to figure out which operations we care about (for example, insert a block at…, split at block at…, replace content at…with a new block, etc…)

Hopefully I will pull together some of my own exploration based on a Rope structure. Let's not be afraid to create Markdown files or code files with example API usage but no actual implementation. I'd rather doodle with code we'll never use than spend a lot of time building the wrong thing 😉

[nylen]

I think we need to do this soon. We have a lot of complicated code around splitting and transforming block types that could be greatly simplified (and unit-tested) with a more appropriate state structure. For example #1674 and #2170 would both benefit from this.

[ellatrix]

Closing the PR (there's still the ticket to discuss). Not sure if we'll eventually go for this approach (as opposed to HTML or tree state), and would need to be redone if so.