Fix, and document, some table/paragraph interrupt corner cases

[notriddle]

Fixes #540

interrupting paragraphs with tables

Pulldown-cmark gives "heavy" tables (with a leading |) higher priority than "light" tables for paragraph interruption. This is compatible with older versions of GitHub, but 2023-06-21 GitHub treats them the same.

A quick run through https://babelmark.github.io shows, of the implementations that support pipe tables at all¹:

Implementation ->	a	b	c	d	e	f	g	h	i	j
pulldown-cmark	✓	x	x	x	x	✓	x	x	✓	x
github/cmark-gfm	✓	x	✓	x	✓	✓	✓	✓	✓	x
pycmarkgfm	✓	x	✓	x	✓	✓	✓	✓	✓	x
DFM2	✓	✓	✓	✓	✓	✓	✓	-	-	x
league/commonmark GFM	✓	✓	✓	✓	✓	✓	✓	x	✓	x
s9e/TextFormatter	✓	✓	✓	✓	✓	✓	✓	x	x	x
markdown-it34	✓	x	✓	x	✓	✓	✓	x	-	-
php-markdown-extra	✓	x	✓	x	✓	✓	✓	x	x	x
markdig	x	x	x	x	x	x	x	x	x	x
MD4C	x	x	x	x	x	x	x	x	x	x
parsedown	x	x	x	x	x	x	x	x	x	x
maruku3	x	x	x	x	x	x	x	x	x	-
cebe	x	x	x	x	x	x	x	x	x	x
pandoc	x	x	x	x	x	x	x	x	x	x
multimarkdown	x	x	x	x	x	x	x	x	x	x

• A lot of Extended Markdown parsers don't allow tables to interrupt paragraphs at all, even if they support pipe tables in general.

• Out of the ones that do, about half of them do what GitHub does and disallow it when the table is indented by four or more spaces.

ambiguity between "light" tables and h2

This PR also changes scan_table_head to refuse any table with zero pipes at all in its header line. This resolves an ambiguity with settext-style <h2>, and matches every other Markdown implementation I could fine in Babelmark (see interrupting paragraphs with tables, "table j"), except maruku and markdown-it, which behave very strangely.

ambiguity with "light" tables and bulleted lists with -

This PR adds a test case for the existing behavior on tables like this:

a | b
- | -
1 | 2

It doesn't change the behavior, but it does document it.

Implementation ->	parsed as
pulldown-cmark	table
github/cmark-gfm	list
pycmarkgfm	list
markdown-it	table
php-markdown-extra	table
DFM	table
league/commonmark GFM	list
s9e/TextFormatter	table
markdig	table
MD4C	list
parsedown	table
maruku	table
cebe	table
pandoc	table
multimarkdown	table

hard line breaks are not allowed at the end of table header lines

Hard line breaks are not allowed at the end of table header lines. It's about a 50:50 split on babelmark, but this change makes pulldown-cmark consistent with GFM and Pandoc.

Implementation ->	parsed as
pulldown-cmark	paragraph
github/cmark-gfm	paragraph
pycmarkgfm	paragraph
markdown-it	table
php-markdown-extra	table
DFM	table
league/commonmark GFM	paragraph
s9e/TextFormatter	table
markdig	paragraph
MD4C	paragraph
parsedown	table
maruku	table
cebe	paragraph
pandoc	paragraph
multimarkdown	table

Footnotes

1. Implementations that "support pipe tables at all" were identified by
   including "table a" without a preceding paragraph and seeing if it
   produced an HTML table for that. ↩

2. On "table h" and "table i", DFM sees a table with no cells, followed
   by a paragraph with b in it. ↩

3. On "table j", maruku and markdown-it see a table with no cells, followed
   by a paragraph with b in it. ↩ ↩²

4. On "table i", markdown-it sees a table with no cells, followed
   by a paragraph with b in it. ↩

[Martin1887]

Thanks!, I will try to review it this week.

[Martin1887]

It looks fine, thanks!