ianstormtaylor deprecate isVoid and related properties/methods (#2102)
#### 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`.)
Latest commit 00d5785 Aug 22, 2018