fix: alignment in pandoc grid tables

[reagle]

Vibe coding using Claude Opus 4.1 thinking to add alignment to pandoc grid tables.

Description

Adds column alignment support for Pandoc grid tables using colon markers in separator rows.

Changes

• Recognize alignment markers in separator rows (:===, ===:, :==:)
• Add PandocAlignColumn and PandocAlignRow classes to handle alignment
• Apply alignment to data cells based on separator row markers
• Maintain full backward compatibility with non-aligned tables

Example

+---------+-------+--------------------+
| Fruit   | Price | Advantages         |
+:========+======:+:==================:+
| Bananas | $1.34 | built-in wrapper   |
+---------+-------+--------------------+
| Oranges | $2.10 | cures scurvy       |
+---------+-------+--------------------+

Left-aligned, center-aligned, and right-aligned columns are now properly recognized and preserved.

[FichteFoll]

I don't have the capacity to review this currently. @keith-hall maybe?

[keith-hall]

I took a cursory look and the changes seem fine. I feel like it should be safe to merge - if something doesn't work, it is easy to revert back to the BorderTable Parser/Driver implementations while this Pandoc implementation gets debugged.