Refine the blank-line handling so a blank line only continues a list run
when the next non-blank line is another list item. This keeps a list ->
paragraph -> list sequence as three separate blocks and matches the
contiguous-list layout for mixed/nested lists (one rich_text block, split
into sub-lists by (indent, ordered)), rather than emitting a separate
block per item.
Adds regression tests for the mixed blank-separated layout and the
list->paragraph->list boundary.
kshitijk4poor
033d7bf259