Foldable fringe icon

[black7375]

One thing that was useful about Magit was that the foldable had an arrow fringe.
This is very useful because you can tell whether it is folded or open.

I saw the icon link in #157 and thought it would be okay to use the following icon.

• https://code.visualstudio.com/api/references/icons-in-labels#icon-listing
  

I briefly looked at the source code to try adding it, but I'm not familiar with the VSCode extension ecosystem, so I'm not sure where to add it.