layout: Scope table row-group children with TableRowGroupBuilder

[servo-wpt-sync]

This pr adds the following changes:

• Introduces TableRowGroupBuilder to handle row-group-scoped traversal more cleanly and explicitly.
• Adds logic to save and restore current_row_group_index, ensuring correct behavior when processing nested row groups.
• Prevents direct traversal of row-group children from the main table traversal flow to avoid incorrect context handling.
• Ensures that anonymous rows created within a row group correctly inherit the row-group context.
• Keeps existing row handling logic unchanged by continuing to delegate row-specific behavior to TableRowBuilder.

Fixes: #43660

Manual test

Run:

Run the following WPT tests to verify the changes:

./mach test-wpt --legacy \
  /_mozilla/mozilla/table_row_group_nested_anonymous_43660.html \
  /_mozilla/mozilla/table_row_group_color_inheritance_43660.html

Reviewed in servo/servo#43674

[wpt-pr-bot]

The review process for this patch is being conducted in the Servo project.

[servo-wpt-sync]

🛠 Changes from the source pull request (servo/servo#43674) can no longer be cleanly applied. Waiting for a new version of these changes downstream.