Add hardLineBreaks option

[Witiko]

When filling the input buffer, TeX removes trailing spaces from the input lines, which prohibits us from detecting line breaks when using markdown in a TeX document (although we can work around this in ConTeXt, see #101). By adding Pandoc's hard_line_breaks syntax extension, we can give users a way to input line breaks even from within TeX documents.

[Witiko]

This feature is now being tested in this private Overleaf document. Note that the current implementation seems to convert text\ntext to text\markdownRendererLineBreak text, but text \ntext to text text. Apparently, there are some interactions between the parsers; future unit tests should focus on this.