Improve BiDi handling of numbers and neutrals.

Add tests for mixed text direction.

Test spanToRuns independently for scripts/directions.

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.

Add expected run lengths to test data.

Add more sample texts.

Deduplicate emptyFont in RunSpec.

Generalise text from SpanData for other tests.

Add Arabic examples to spanToRuns tests.

Consolidate tree types into one.

Implement finding box edges.

Add required textDirection to TextOptions.

Use ResolvedBox when flattening trees.

Implement BiDi resolved level reordering.

Fix style: long lines.

Fix style: unnecessary do notation.

Fix style: white space in lambdas.

Fix style: white space around operators.