List: Add prop for line-height

[sarahmonster]

We may want to give the option for different line-heights (possibly for other text elements as well?)

Right now, there's a 1.6rem margin at the bottom of each list element, which works well when list items span more than one line, but can feel excessive when list items are shorter.

It would be good to provide a theme-configurable prop for this, something like: spacing: tight or line-height: tight.