Add a `verse/presep` option to increase the spacing before a verse.

If you set `verse/sep` to a larger value (e.g., 0.5em) then the normal space before the verse appears unnaturally small.

Add a `verse/presep` option to set this. It could default to 0em and only needs to be used for normal non-overlapping verses (e.g., not in poetry and beginning of paragraphs in `hanging` environments.