Update isHistory to match new interface. (#5197)

* Update `isHistory` to match new interface.

This change merely preserves the original functionality and does not add verification of Batch's new `selectionBefore` property.

* Add changeset.

packages/slate-history/test/isHistory/after-edit.js

@@ -0,0 +1,18 @@
+/** @jsx jsx */
+
+import { Transforms } from 'slate'
+import { jsx } from '..'
+
+export const input = (
+  <editor>
+    <block>
+      Initial text <cursor />
+    </block>
+  </editor>
+)
+
+export const run = editor => {
+  Transforms.insertText(editor, 'additional text')
+}
+
+export const output = true