Introduce keyboard navigation in context menus

crates/menu/src/menu.rs

@@ -0,0 +1,16 @@
+#[derive(Clone)]
+pub struct SelectIndex(pub usize);
+
+gpui::actions!(
+    menu,
+    [
+        Cancel,
+        Confirm,
+        SelectPrev,
+        SelectNext,
+        SelectFirst,
+        SelectLast
+    ]
+);
+
+gpui::impl_internal_actions!(menu, [SelectIndex]);