Closes #ISSUE

```json
  {
    "context": "OutlinePanel && not_editing",
    "bindings": {
      "enter": "editor::ToggleFocus",
      "/": "menu::Cancel",
      "ctrl-u": "outline_panel::ScrollUp",
      "ctrl-d": "outline_panel::ScrollDown",
      "z t": "outline_panel::ScrollCursorTop",
      "z z": "outline_panel::ScrollCursorCenter",
      "z b": "outline_panel::ScrollCursorBottom"
    }
  },
  {
    "context": "OutlinePanel && editing",
    "bindings": {
      "enter": "menu::Cancel"
    }
  },
```

Release Notes:

- Added scroll keybindings for the OutlinePanel

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>