I don't think there is a way to directly change the interline spacing but I think you can accomplish what you want like this.
go to the options dialog, and look in the schemes section.
edit the scheme for the type of file you will be editing
In the styles tab each text style can have an independent font/size/color etc. the Scintilla control that pn2 is based on spaces all the lines based on the largest font in its list. So if you use a 10 point font as your standard editing font and you change one of the font styles to an 11 point font,(I used UUID) Scintilla will space everything out to 11 points giving a bit more space between lines.
If a full point jump in spacing is too much you would have to experiment with other fonts to find one that gives an interline spacing you like.
This isn't as easy as a direct adjustment but a direct adjustment would be very difficult given the really flexible font control that pn2/scintilla provides.