Add line_widths to Prawn::Text::Formatted::Box

[ekkxl]

Add line_widths to Prawn::Text::Formatted::Box, which holds an Array with the width of each line. This is useful with the dry_run option to determine how wide text will be, so that, for example, one could calculate some layout before rendering the text.

Unlike Document#width_of, this goes through the whole rendering pipeline, so it handles things like fallback fonts. It also handles multiple lines directly.