Add GFM Table Serialization Support to mdast_util_to_markdown

[jp-knj]

Closes: #171

Overview

This PR implements table serialization support for the mdast_util_to_markdown crate, bringing it to feature parity with the JavaScript https://github.com/syntax-tree/mdast-util-gfm-table package. Previously, attempting to serialize ASTs containing GFM table nodes would result in an "unexpected-node" error.

Testing

Added comprehensive test coverage in tests/table.rs

• Basic table structures
• Column alignment variations
• Unicode content handling
• Pipe escaping scenarios
• Empty cells and edge cases
• Complex inline elements
• Error conditions

Run tests with:

# Build with the feature
cargo build --features unicode-width

# Run tests with the feature
cargo test --features unicode-width

Uses conditional compilation to:

• Default: Character count (.chars().count()), no dependency needed
• With feature: Unicode display width, proper CJK/emoji alignment

[wooorm]

unicodewith seems like a step way from the upstream. And, a dependency.
Why not make it optional, configurable?

[jp-knj]

You're right, I'll do that