keymap: change to support hard-breaks

[gyger]

Allows for Shift-Enter to create a hard break. This follows the implementation of the prosemirror sample. https://github.com/ProseMirror/prosemirror-example-setup/blob/master/src/keymap.ts

[gyger]

This is a different way to implement the support for Shift+Enter for hard breaks in the editor. Similar to PR https://github.com/cosmocode/dokuwiki-plugin-prosemirror/pull/115, but this follows the implementation of the example editor.