Tables: Implement nested tables (!===)

[nlopes]

Problem

Nested table syntax using !=== separators is not supported.

AsciiDoc syntax

[cols="1,2"]
|===
| Outer Cell 1
a|
[cols="1,1"]
!===
! Inner A ! Inner B
! Inner C ! Inner D
!===

| Outer Cell 2
| Simple content
|===

The !=== delimiter and ! cell separator allow tables inside AsciiDoc cells (a|).

Current behavior

The !=== syntax is not recognized. Inner table content is treated as literal text.

Implementation requirements

1. Parser changes

• Recognize !=== as nested table delimiter in parse_rows_with_positions()
• Track nesting depth to handle ! vs | cell separators correctly
• Recursive parsing for table-in-cell content

2. AST considerations

The current model already allows Vec<Block> in table cells, so nested Table blocks should work once parsing is fixed.

3. HTML renderer

Should already handle nested tables since it walks block content recursively. May need testing.

Workaround

• Flatten table structure
• Use description lists for hierarchical data
• Use multiple separate tables

Priority

Low - rare use case in practice.

References

• https://docs.asciidoctor.org/asciidoc/latest/tables/nested-tables/