Fix a blank line with partial indent evicting a list item early

[afonsojanu]

Fixes #618.

A blank line inside a list item made up only of spaces short of the item's own indent width would end the item early and push whatever came after out as a sibling paragraph, even though a fully empty blank line or one padded out to the full indent width kept the item open.

The three shapes from the issue only diverge when the item's opening line is a link reference definition on its own. Once the reference resolves, finalize() frees that now-empty paragraph, and parse_node_item_prefix was using container->first_child == NULL as a stand-in for "the opening line was blank", since normally that's the only way an item ends up childless at this point. A reference-only opening line hits the same childless state for a different reason entirely, so a later short blank line got misread as "this item never had anything in it." A blank line with zero or full-width indent happens to avoid the branch in parse_node_item_prefix that even looks at first_child, which is why only the partial-indent case showed the bug.

Added a CMARK_NODE__ITEM_HAD_CONTENT flag, set on the item when a reference-only paragraph belonging to it gets freed, and checked alongside first_child. Added a regression test using the exact input from the issue.

Ran the full local suite (api tests, CommonMark spec tests, smart punctuation tests, regression tests) before and after. Confirmed the new case fails without the blocks.c change (reverted it locally, rebuilt, saw the failure) and passes with it restored.