Add paragraphLines to Rich ParagraphLayout.
BREAKING: The paragraphLines field of ParagraphLayout must be preserved
for pagination.
BREAKING: The width of a rich text ParagraphLayout will now remain
unchanged after pagination.
BREAKING: Empty space at the top of the first line and at the bottom
of the last line in a ParagraphLayout will now be preserved.
Add internal structure for vertical offsets.
Test paragraphSafeWidth on LTR paragraphs.
Refactor lineNumbers into a common module.
Add centre/right paragraph alignment.
Separate split list functions into a module.
Convert legacy span directions into levels.
Move BiDi Level to separate module.
Add helper function for run-length encoding.
Generalise text from SpanData for other tests.
Prepare for extending ProtoRun.
Consolidate tree types into one.
Handle ancestor boxes in rich layout.
Add ancestor boxes to interface.
Implement finding box edges.