Support hard line breaks in Content Editor

What does this MR do?

Adds support for hard line breaks in the Content Editor. This node type is part of the Commonmark specification.

• Commonmark specification
• The content editor will always represent hard line breaks by appending a \ at the end of a line.
• The other representation of hard line breaks is adding two or more spaces at the end of the line. The content editor will convert this implicit representation into a \.

Screenshots (strongly suggested)

This MR does not introduce user-facing changes.

Does this MR meet the acceptance criteria?

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