settings_ui: Make arrow keys up and down activate page content (#40106)

Release Notes:

- settings ui: Navigating the settings navbar with arrow keys up and
down now also activates the page, allowing users to more quickly see the
content for a given page before moving focus to the page itself.

assets/keymaps/default-linux.json

@@ -1249,6 +1249,7 @@
       "escape": "workspace::CloseWindow",
       "ctrl-m": "settings_editor::Minimize",
       "ctrl-f": "search::FocusSearch",
+      "left": "settings_editor::ToggleFocusNav",
       "ctrl-shift-e": "settings_editor::ToggleFocusNav",
       // todo(settings_ui): cut this down based on the max files and overflow UI
       "ctrl-1": ["settings_editor::FocusFile", 0],
@@ -1269,6 +1270,8 @@
     "context": "SettingsWindow > NavigationMenu",
     "use_key_equivalents": true,
     "bindings": {
+      "up": "settings_editor::FocusPreviousNavEntry",
+      "down": "settings_editor::FocusNextNavEntry",
       "right": "settings_editor::ExpandNavEntry",
       "left": "settings_editor::CollapseNavEntry",
       "pageup": "settings_editor::FocusPreviousRootNavEntry",