Interpret a `split-block` as format toggle for an empty block
Interpret a `split-block` as format toggle for an empty block

(`split-block` is emitted when "return" is pressed)

part of fixing https://github.com/vector-im/riot-web/issues/4580

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk