Split out slate-dom package from slate-react

[bmingles]

Problem
It looks like the slate-react package contains a fair amount of non-react code. Conceptually it's more like a combination of DOM + React specific features. It would be nice if the DOM only utils lived in a package that didn't have a dependency on React at all. This could make it easier for non-React web libraries to build on top of slate without re-inventing the DOM parts.

Solution
A high level proposed solution would be to:

• Create a new slate-dom package in the monorepo
• Move src/utils from slate-react to slate-dom. There are a very few things that would need to stay such as some React constants, but for the most part there are very few React dependencies in this folder.
• Move most of plugin/react-editor to the new package and rename to plugin/dom-editor. There is a part of the onChange implementation that would still need to be implemented by slate-react. The react-editor plugin could now call the dom-editor plugin for most of its implementation and fill in anything remaining that is React specific
• commonTypes to be split into React vs DOM specific concerns and live in respective package

Components would stay in slate-react, although there may be some places where logic could be refactored and made suitable for the slate-dom package. e.g. portions of the event handling code in Editable for onInputChange and onSelectionChange. There are likely some other places as well, but the above bulleted list could be a good starting point to make it easier for library authors.

I'd be interested in submitting a PR for this, but I wanted to get a general sense if this would be a welcome improvement, or if it is fundamentally flawed somehow in relation to goals of Slate.

[bmingles]

@dylans very happy to see this. When should the npm publish of the new package happen?