Make canJoin return false for all leaf nodes
Make canJoin return false for all leaf nodes

salzhrani/prosemirror,tzuhsiulin/prosemirror,salzhrani/prosemirror,salzhrani/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror