settings_ui: Improve keyboard nav (#39819)

Closes #ISSUE

From notes:

```markdown
  - [x] Clicking on the disclsoure icon button in the root-level tree view item should steal focus and move it to the root item (not the icon button)
  - [x] [@ben] Allow left/right arrow keys to expand/collapse root tree view items in the nav
    - [x] With this, make enter/space work the same as clicking (activate page, don't expand root items, focus moves to the content and leaves nav — becomes consistent with mouse interaction)
  - [x] Smart cmd-shift-e: toggling focus should take you to the selected item
  - [x] [@ben] pageup + pagedown in nav -> jump between root items
  - [x] [@ben] home + end buttons should work
    - in nav:
      - home always goes to first section header
      - end always goes to last _visible_ item (does not expand)
```

Release Notes:

- N/A *or* Added/Fixed/Improved ...

assets/keymaps/default-linux.json

@@ -1264,5 +1264,17 @@
       "ctrl-pageup": "settings_editor::FocusPreviousFile",
       "ctrl-pagedown": "settings_editor::FocusNextFile"
     }
+  },
+  {
+    "context": "SettingsWindow > NavigationMenu",
+    "use_key_equivalents": true,
+    "bindings": {
+      "right": "settings_editor::ExpandNavEntry",
+      "left": "settings_editor::CollapseNavEntry",
+      "pageup": "settings_editor::FocusPreviousRootNavEntry",
+      "pagedown": "settings_editor::FocusNextRootNavEntry",
+      "home": "settings_editor::FocusFirstNavEntry",
+      "end": "settings_editor::FocusLastNavEntry"
+    }
   }
 ]