Normalize copying void inline nodes (#2198)

When a void inline nodes is selected along with other text/content it can be copied to the clipboard. When it’s the only thing selected it is ignored because anything marked contentedtiable=false is.
Updated isInEditor to return true if a contenteditable=false has a parent that is a void inline node.

packages/slate-react/src/components/content.js

@@ -284,8 +284,10 @@ class Content extends React.Component {
 
       throw err
     }
+
+    const allowEdit = el.isContentEditable || el.closest('[data-slate-void]')
     return (
-      el.isContentEditable &&
+      allowEdit &&
       (el === element || el.closest('[data-slate-editor]') === element)
     )
   }