Add options for vertical alignment.

Add content rect to Fragment.

Add helper function to strut empty boxes.

Expose paragraphSafeWidth.

Allow explicit control of box collapsing.

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.

Remove paragraphLineHeight from Rich interface.

Expose rect unions in public interface.

Describe box edges with extensible type.

Expose piecewise paragraph construction.

Add ancestor boxes to interface.

Add required textDirection to TextOptions.

Deprecate ParagraphConstruction.

Deprecate the Plain interface.

Add public Rich interface.

Add user data to spans.

Add line numbers to laid out fragments.

Qualify Plain interface.

Future-proof records with options.