Added the Commonmark backslash linebreak

[JeroenVerseput]

I noticed that de dubble whitespace linebreak did not work. After some research i found out that de backslash was also a valid method to create a linebreak. So i used the existing switch case to check if the LineBreakInline was created because of a backslash.

I hope this could be an addition too this solution.

Source i found: https://commonmark.org/help/tutorial/03-paragraphs.html

[christiaanderidder]

Hi @JeroenVerseput, thank you for your submission. Looks like a useful addition to me.