[folding] seperate color for line numbers at collapsed row

When we collapse rows, it's difficult to tell that we have, especially since hovering over the line numbers gutter causes all of the expand/collapse indicators to return. I realize that, currently, a sticky expand indicator is left behind when rows are collapsed, but I propose that the line numbers themselves be lit up with a themable color. So, for example, if rows collapse so that we have ... 149 150 **151 306** 307 308 ... the numbers **151** and **306** would glow brighter in some way. This would be in addition to the sticky expander.