Generalise text from SpanData for other tests.

Add Arabic examples to spanToRuns tests.

Consolidate tree types into one.

Implement finding box edges.

Add required textDirection to TextOptions.

Use ResolvedBox when flattening trees.

Implement BiDi resolved level reordering.

Fix style: long lines.

Fix style: unnecessary do notation.

Fix style: white space in lambdas.

Fix style: white space around operators.

Fix style: white space in multi-line lists.

Fix style: white space in exports and imports.

Fix style: bullet points in comments.

Mark some lists as explicitly non-empty.

Implement generic pagination internally.

Test removing spaces around lines.

Remove collapsible spaces at beginning of lines.

Allow empty lines when hard line breaking.

Reduce test dependencies on fonts.