Expand context around hunks #2

Something like how GitHub/Forgejo/et al let you click some kind of up/down arrow near the gutter to see more code above/below the hunk. Just a keybind and visual indicator for "this is expandable" for now, then we can think about how to expose it through a clickable button thing later. Should support comments and normal navigation and all that stuff. Maybe also include some way to collapse it again? I worry that would risk adding too many knobs though. I think GH only lets you expand and I've found that annoying and would like to collapse, but collapse UX complicat.