fix hard line break round-trip in commonmark renderer

[zayeem06]

The commonmark renderer writes hard line breaks as two trailing spaces, but those are only recognized as a hard break when they follow other text on the line. When a hard break begins a line (a leading \ break, or one right after a soft break) or comes after a space, the spaces are stripped as insignificant trailing whitespace and the break, along with any preceding space, is lost when the output is parsed again. This emits a backslash line break in those positions and keeps the two-space form everywhere else; the new api_test cases cover the three positions and the roundtrip tests exercise it end to end.

[jgm]

I'd prefer just to emit the backslash everywhere for hard breaks.

[zayeem06]

Makes sense, that's simpler. Updated it to always emit the backslash break and dropped the positional check; tests still pass.