#### Is this adding or improving a _feature_ or fixing a _bug_?
Improvement.
#### What's the new behavior?
This deprecates the `node.isVoid` property in favor of using `schema.isVoid(node)`, which will allow us to remove the hardcoded "void" concept from the data model, and have it depend on the schema instead.
This allows you to build different kinds of editors, with different void semantics, depending on your use case, without having this information hardcoded in the data itself. Even switching the `isVoid` semantics on the fly based on a user toggling a setting for example.
#### How does this change work?
This is the first step, which just deprecates `node.isVoid`, and provides the new alternative of `schema.isVoid(node)`, while still using the `isVoid` value of nodes under the covers.
The next step is to change the logic to search the schema for real, and completely remove the `isVoid` value from nodes.
#### Have you checked that...?
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* [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`.)
* Fix spell check bug by add data-text:true
* Fix spell check bug by spell check add length to a leaf
* Fix tests to use data-text:true for marks
* Rename data-text to data-slate-leaf; Remove setRef; unlift attributes in leaf
* Update examples with data-*
* Add attributes to document
* Fix renderMark in all documents
* Prettier markdown
* Set up webpack configuration for building examples
* Configure react-hot-loader in development
* Improve config, set gh-pages to use examples/dist directory
* PR feedback
* Rename App.js in git
* 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
* remove data from event handler signatures
* standardize known transfer types
* add setEventTransfer to docs
* update examples, fix drag/drop
* fix tests and draggable attribute setting
* refactor to extract applyOperation util
* change handlers to receive transform instead of state
* change onChange to receive a transform, update rich-text example
* fix stack iterationg, convert check-list example
* convert code-highlighting, embeds, emojis examples
* change operations to use full paths, not indexes
* switch split and join to be recursive
* fix linter
* fix onChange calls
* make all operations invertable, add src/operations/* logic
* rename "join" to "merge"
* remove .length property of nodes
* fix node.getFragmentAtRange logic
* convert remaining examples, fix existing changes
* fix .apply() calls and tests
* change setSave and setIsNative transforms
* fix insert_text operations to include marks always
* cleanup and fixes
* fix node inheritance
* fix core onCut handler
* skip constructor in node inheritance
* cleanup
* change updateDescendant to updateNode
* add and update docs
* eliminate need for .apply(), change history to mutable
* add missing file
* add deprecation support to Transform objects
* rename "transform" to "change"
* update benchmark
* add deprecation util to logger
* update transform isNative attr
* fix remaining warn use
* simplify history checkpointing logic
* fix tests
* revert history to being immutable
* fix history
* fix normalize
* fix syntax error from merge
* WIP getting raw marks arrays from the current range
* Always handle marksRaw as Array, fixes tests/lint
* Clean up
* Fixes collapsed selection raw marks, simpler _every_ condition, harmonize toolbar buttons in rich-text example
* raw -> perCharacter
* Add tests for toggleMark collapsed selection
* Add .DS_Store to .gitignore
* Added test for toggleMark add in partially marked selection, with and without other marks
* Added docs for state.marksPerCharacter
* replace marksPerCharacter with activeMarks
* Update the other examples
* Clarify getActiveMarksAtRange
* AddMark/RemoveMark to use getActiveMarksByRange
* User activeMarks for toggle§MarkAtRange transform
* Add property isEmpty to State
* Update hovering menu example
* Document isEmpty
* Improve perf of isEmpty with @Soreine 's suggestion
* Fix return of isEmpty
