Optimize isText, isElement, isNodeList and isEditor (#5859)

* Remove the `isPlainObject` check from `isText` and `isElement` for performance

* Optimise `isElement`, `isNodeList` and `isText` further

* Update changeset

* Fix changeset

* Refactor object check into `isObject`

packages/slate-react/package.json

@@ -17,7 +17,6 @@
     "@juggle/resize-observer": "^3.4.0",
     "direction": "^1.0.4",
     "is-hotkey": "^0.2.0",
-    "is-plain-object": "^5.0.0",
     "lodash": "^4.17.21",
     "scroll-into-view-if-needed": "^3.1.0",
     "tiny-invariant": "1.3.1"