[Article editor] `SHIFT` + `ENTER` should insert a new line, and not insert a new block

[ciphrd]

• base branch: feature/articles

When the cursor is in the following blocks:

• p
• ul/ol
• blockquote

Pressing SHIFT + ENTER should insert a new line within the block

[maerzhase]

Surprisingly the editor exposes an API for softbreak

  insertSoftBreak: () => void

https://docs.slatejs.org/concepts/07-editor