Consolidate tree types into one.

Test interaction between boxes and mixed script.

Test soft breaking on box spacing.

Test line breaking in boxes.

Add ancestor boxes to interface.

Implement finding box edges.

Make paragraph direction explicit in tests.

Store direction in ResolvedBox.

Add required textDirection to TextOptions.

Link ResolvedSpan to boxes.

Use ResolvedBox when flattening trees.

Test layout with mixed font sizes.

Test Rich pagination.

Test Rich line wrapping.

Test Rich layout.

Limit internal imports in interface tests.

Make paginateAll generic.

Deprecate the Plain interface.

Embed TextOptions in ResolvedSpan.

SpanOptions are relegated to the legacy Plain interface, and
TextOptions are now being used internally.

Add user data to spans.