* Support the Node linebreakReplacement property
Support the [linebreakReplacement](https://prosemirror.net/docs/ref/#model.NodeSpec.linebreakReplacement) property on Nodes, allowing a node to be used wherever a newline character is, when converting between blocks that don't support the linebreak node, but have their whitespace set to "pre".
This is useful, for example, when converting between code blocks & normal paragraphs. Code blocks don't support the linebreak node, but do allow newline characters ('\n').
Marking the `hardBreak` node as the `linebreakReplacement` will mean the newline characters within codeBlock nodes will be converted to `hardBreak` nodes in the paragraph.
* Make hardBreak the default linebreakReplacement node
When converting between codeBlocks and normal paragraphs, the hardBreak node should be used in place of newline '\n' characters.
