Fix style: white space in exports and imports.

Mark some lists as explicitly non-empty.

Separate module for the ParagraphLayout type.

Remove collapsible spaces at beginning of lines.

Allow empty lines when hard line breaking.

Support hard line breaks.

Document considerations for beginsText/endsText.

Properly use ICU locale identifiers.

Clarify spanLanguage and add basic ICU conversion.

Refactor line breaking.

Allow breaking at character bounds.

Use a single list of line breaking alternatives.

Implement trimming spaces at end of lines.

Remove redundant overflow handling.

This may be useful later when words are allowed to overflow, but this
code path is currently unused.

Decompose ProtoFragment construction.

Implement correct positioning of RTL runs.

Make SpanOptions a separate type.

Slightly decompose the Plain module.

Update documentation and derived instances.

Use internal library to hide non-public modules.