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.

Describe box edges with extensible type.

Expose piecewise paragraph construction.

Add ancestor boxes to interface.

Add required textDirection to TextOptions.

Add public Rich interface.