#### Is this adding or improving a _feature_ or fixing a _bug_?
Improvement.
#### What's the new behavior?
This introduces two new models: `Decoration` and `Selection`, which both implement the simpler `Range` interface. This way we can introduce properties to these concepts without having to have them live on all ranges, and we can start to introduce more helpful methods specific to each one's needs.
It also means we don't need to move `isFocused` to value, which saves some complexity on the operations side, retaining `set_selection` as the only way selections are modified.
In the process, it also cleans up a lot of the existing model logic for implementing the `Node` interface, and introduces another `Common` interface for shared properties of all Slate models.
#### How does this change work?
It introduces a new `interfaces/` directory where common sets of properties can be declared, and mixed in to the models with the new (simple) `mixin` utility.
#### Have you checked that...?
* [x] The new code matches the existing patterns and styles.
* [x] The tests pass with `yarn test`.
* [x] The linter passes with `yarn lint`. (Fix errors with `yarn prettier`.)
* [x] The relevant examples still work. (Run examples with `yarn watch`.)
#### Does this fix any issues or need any specific reviewers?
Fixes: #1952Fixes: #1807
Fixes: https://github.com/ianstormtaylor/slate/issues/2110
* Add Prettier, with basic config and ESLint integration
* Apply Prettier to all files using `yarn lint --fix`
* Tell Prettier to ignore an empty text in a test output.
* Run Prettier on JS files not handled by ESLint, and lint them too
* rename state to value in slate core, as deprecation
* rename all references to state to value in slate core
* migrate slate-base64-serializer
* migrate slate-html-serializer
* migrate slate-hyperscript
* migrate slate-plain-serializer
* migrate slate-prop-types
* migrate slate-simulator
* fix change.setState compat
* deprecate references to state in slate-react
* remove all references to state in slate-react
* remove `value` and `schema` from props to all components
* fix default renderPlaceholder
* fix tests
* update examples
* update walkthroughs
* update guides
* update reference
