Store default level in BiDiLevels.

This will be required for handling empty text sequences.

Improve BiDi handling of numbers and neutrals.

Autodetect text direction for legacy interface.

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.

Convert legacy span directions into levels.

Move BiDi Level to separate module.