Support standard markdown hard line breaks

[runebrg]

Hi
Currently the editor doesn't seem to support hard line breaks the CommonMark way. That is, a double space or a backslash before a linebreak should be rendered as a <br/>.

https://spec.commonmark.org/0.29/#hard-line-breaks

Example using a plain textarea, the MarkDownIt renderer and rich-markdown-editor:

As you can see, rich-markdown-editor does not break the lines properly, but adds two linebreaks before the text instead