Converting a code block with newlines to a paragraph creates a paragraph with newlines

Which are visible, but then normalized back to spaces as soon as you type something and the paragraph is re-parsed.

Ideally, the newlines should be converted to hard breaks when the textblock type is changed, though it's going to be kind of tricky to find a reasonable place for that code (it's very schema-specific).

Via https://github.com/ProseMirror/prosemirror-markdown/issues/26