fix: nested lists, and a github flavour for admonitions

[auscaster]

Closes the two remaining reports from @gkodinov.

Nested lists (fixes #115)

Both halves of his report, and the ordered-list case he did not mention:

• Every list item was followed by a blank line, because a paragraph inside a list item closed as its own block.
• Nested lists were flattened, so a two-level list rendered as seven bullets at one level.

Indentation is measured against the parent item's marker rather than a fixed width, so a list nested under 1. indents three spaces and one under * indents two. That is what Markdown needs to actually nest, and hardcoding two spaces would have quietly broken ordered lists.

The three list kinds had three identical case bodies; they are one now.

GitHub alerts (fixes #126)

--flavour github renders admonitions as GitHub alerts:

> [!NOTE]
> Flushing is best effort.

> [!WARNING]
> Blocks until the queue is empty.

@note and @remark map to [!NOTE], @warning to [!WARNING], @attention to [!IMPORTANT]. Default stays pandoc and its output is byte-identical: the two dialects now share one renderer, so the pandoc path could not drift while adding the other.

Worth knowing: no fixture had a single admonition, so the :::note path had never been rendered by a test. It has coverage in both dialects now.

Also note @deprecated reaches us as an xrefsect rather than a simplesect, so it still renders as a plain > Deprecated: … blockquote in both flavours. Mapping that would mean matching on a localised title string, which I would rather not build.

No version bump. Nothing releases until you say so.