Store parent text options in ResolvedBox.

This is required for vertically aligning the box with its parent.

Store default level in BiDiLevels.

This will be required for handling empty text sequences.

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.

Implement finding box edges.