[expand button]: think about better choice of the arrow icon

[legomushroom]

@philliphoff comment:

I'm not crazy about the icon choice; it's not obvious to me what the expand/collapse icon mean in this context. To me, using + and - might have been more clear, or icons that are clear arrows.

[legomushroom]

For context, this is modeling the expand/collapse behavior and UI in VS Code (find/replace) for now, but yeah, we can revise this.

I'd say it still doesn't work great in VS Code, but at least works slightly better since it's not expanding to a large panel. I think these icons would be more obvious: