Integrate BiDi levels with spanToRuns algorithm.

Span/box boundaries no longer reset directionality.

BREAKING: Text with no strongly directional characters would
previously be treated as having no direction. It will now be
treated as having the direction defined in TextOptions.
This affects calculated positions.

Add shaped runs to manually test pagination.

Describe box edges with extensible type.

Test interaction between boxes and mixed script.

Test line breaking in boxes.

Handle ancestor boxes in rich layout.

Add ancestor boxes to interface.

Test layout with mixed font sizes.

Test Rich pagination.

Test Rich line wrapping.

Test Rich layout.

Add user data to spans.

Add line numbers to laid out fragments.

Update formatting in golden files.

All golden files are regenerated to use the new formatting.

This only affects whitespace (including newlines), otherwise the golden
values are perfectly equal. You can check this using the command:
`git show --word-diff-regex='[^[:space:]]'`

Rename golden files for easier tracking.

Make demo font size better match window.

Add paragraph pagination.

Test removing spaces around lines.

Allow empty lines when hard line breaking.

Support hard line breaks.