Separate internal plain Paragraph/ParagraphLayout.

Expose text slicing functions.

Fix style: white space in exports and imports.

Prepare pagination for future layout types.

Add paragraph pagination.

Separate module for the ParagraphLayout type.

Make SpanOptions a separate type.

Slightly decompose the Plain module.

Update documentation and derived instances.

Use internal library to hide non-public modules.

Move non-public modules into Internal namespace.

Implement "plain" interface.

Separate modules.

WIP version with WIP interface.