HighlightBoth(ReadOnlyMemory<Char>, String?) from the state the previous line ended in: see
Highlight(ReadOnlyMemory<Char>, String?, ref Object?). The default ignores
state.
IReadOnlyList<(string Text, IBrush? Dark, IBrush? Light)>? HighlightBoth(ReadOnlyMemory<char> line, string? language, ref object? state)