Generalise shiftFragment function.

Intended for testing paragraphSafeWidth.

Refactor lineNumbers into a common module.

Fix documentation: internal references.

Deduplicate pagination code.

Implement Rich pagination.

Refactor pagination to allow rich layout.

Add user data to spans.

Use line numbers for page breaks.

Fix documentation: export dependencies of exported names.

Separate internal plain Paragraph/ParagraphLayout.

Fix style: white space in lambdas.

Implement splitting ParagraphLayout into lines.