Add up and down global bindings for menu::SelectNext and menu::SelectPrevious (#32981)

Closes https://github.com/zed-industries/zed/discussions/32587

Release Notes:

- Fixed Linux keybindings not having up/down keys bound for menu
navigation

assets/keymaps/default-linux.json

@@ -10,8 +10,10 @@
       "pagedown": "menu::SelectLast",
       "ctrl-n": "menu::SelectNext",
       "tab": "menu::SelectNext",
+      "down": "menu::SelectNext",
       "ctrl-p": "menu::SelectPrevious",
       "shift-tab": "menu::SelectPrevious",
+      "up": "menu::SelectPrevious",
       "enter": "menu::Confirm",
       "ctrl-enter": "menu::SecondaryConfirm",
       "ctrl-escape": "menu::Cancel",