fix(convert): render a block inside a list item as a block

[willkg]

Fixes #211.

read/export sent every <li> child other than a <p> or a nested list through renderInline, so a code macro in a list item came out with its opening fence on the item's text line and its body unindented. The Confluence editor writes exactly that shape (<li>text<ac:structured-macro ac:name="code">…</li>), and SREIN Triage and Work Process has two of them. This is older than the line-break/paragraph work: renderListItem was unchanged from v0.1.0 until this PR.

What changes

• A code block, <pre>, table, quote, callout or heading inside a list item now renders as a block indented under the item. Nested lists and blocks keep their document order.
• A fence needs no blank line next to text, another fence or a nested list, so an item with text and a code block stays a tight list and publishes back to the storage it came from. Every other block gets a blank line, which it needs: a quote would swallow the text after it, a raw HTML table would swallow a fence, and text after a nested list would continue that list's last item.
• Loose inline text in an item now renders as one run through renderInlineChildren. Text after a <br /> no longer gains a space on every round trip, and a mark split around a link is repaired the way it already is in a <p>.
• <hr> is deliberately not handled as a block. - --- is a thematic break, not a list item, and the editor has no way to put a divider in a list.

Testing

• New testdata/storage2md/list-blocks case covering the SREIN shape, the editor's <p> form, text after a block, an item that is only a code block, nested lists before and after blocks, a raw table followed by a fence, callouts, quotes, tables, and an inline status macro that must stay inline. Being a storage2md case, it also runs through TestRoundTripMarkdownIsAFixedPoint and the output-parses-as-Markdown check.
• No other golden changed.
• Ran against the live page: exporting it now differs from v0.2.0 only at the two code blocks, and check --show-html on the new export reproduces the page's original <li>text<ac:structured-macro …> storage.
• make check passes.

Review findings left out (older, not caused by this change)

• Text after a block that starts with Markdown block syntax (- , # ) is read back as that block. This is the read: paragraph text that looks like a Markdown block becomes structure on publish #203 class, now also reachable inside a list item.
• A code macro nested inside a <p> still goes through the inline path. Confluence doesn't write that shape.
• A block that needs a blank line makes the whole list loose, so sibling items publish with <p> around their text. This is CommonMark's rule, and the Markdown is still a fixed point.
• Consecutive <p>s in one item are still joined with a space.