Add start/end paragraph alignment.

In the future, when the paragraph direction is not explicitly set,
start/end options will allow alignment to change line by line,
whereas left/right alignment will be constant.

Add centre/right paragraph alignment.

Internally mark hard line breaks.

Make RTL text overflow the left paragraph edge.

This behaviour is consistent with CSS Text Module Level 3.

BREAKING: Previously, all text would overflow the right paragraph edge,
regardless of text direction.

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.

Link ResolvedSpan to boxes.

Separate WithSpan for sharing with other modules.

Add basic support for Rich layout.