In a folded block scalar, a blank run of N lines between two non-more-indented
lines folds to N line breaks: the fold absorbs the first break. Next to a
more-indented line nothing folds, so that break survives and the run yields
N+1 breaks. The folder used N in every case, so paragraph spacing around an
indented block collapsed: a blank line before a more-indented block came out
as one break instead of two.
Track whether the line before the blank run, or the one after it, is
more-indented and add the extra break in that case. Folding between plain
lines and runs with no blank lines are unchanged.
Raises yaml-test-suite value-level conformance to 186/194 (95.9%) with the
parse-level and should-error rejection rates unchanged. 444 tests pass.