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 manual test for mixed vertical alignment.

Test shifted baseline.

Test mixed line height with varied alignment.

Test preservation of lines with boxes.

Test boxes with no text or only a space.

Test paragraphSafeWidth on LTR paragraphs.

Test with AlignStart by default.

Test centred paragraph.

Test right-aligned paragraph.

Test handling of empty lines.

Test mixed line height.

Test shaping across intra-word breaks.

Test with only directionally neutral characters.

Add shaped runs output for bidirectional text.

Add tests for mixed text direction.

Test interaction between boxes and mixed script.

Test soft breaking on box spacing.

Test line breaking in boxes.

Test layout with mixed font sizes.