Avoid backslashes before a new block.

[jneem]

This changes the behaviour of the commonmark renderer to avoid rendering newlines as backslashes (1) at the end of a block and (2) when the next node in the current block is a block.

Fixes #372.

This isn't quite the solution that I described in the comment, because I discovered another situation where backslash-newlines are treated differently from the double-space variant:

a
\
b

is parsed as a single paragraph with a line break, but the double-space version is parsed as two paragraphs with no line break. I think the backslash variant is the one most likely to round-trip correctly, so I went with that as often as possible.

[kivikakk]

This looks awesome; thank you so much! 🤍

If either of @digitalmoksha or @gjtorikian are able to give it a once over, I would much appreciate it! But it looks like a very nice and straightforward improvement.