Fix for backspace press in list (#3533)

Made a fix for the fact that if you were standing in a <li> wrapped in an <ul> with no text and pressed backspace, the <ul> would go away while the <li>'s stay.

site/examples/markdown-shortcuts.js

@@ -96,6 +96,7 @@ const withShortcuts = editor => {
           if (block.type === 'list-item') {
             Transforms.unwrapNodes(editor, {
               match: n => n.type === 'bulleted-list',
+              split: true,
             })
           }