Navigation: Refactor mobile menu into it's own component (#41308)

* Navigation: Start creating new NavBarMenu component

* Navigation: Apply new NavBarMenu to NavBarNext

* Navigation: Remove everything to do with .sidemenu-open--xs

* Navigation: Ensure search is passed to NavBarMenu

* Navigation: Standardise NavBarMenuItem

* This extra check isn't needed anymore

* Navigation: Refactor <li> out of NavBarMenu

* Navigation: Combine NavBarMenuItem with DropdownChild

* use spread syntax since performance shouldn't be a concern for such small arrays

* Improve active item logic

* Ensure unique keys

* Remove this duplicate code

* Add unit tests for getActiveItem

* Add tests for NavBarMenu

* Rename mobileMenuOpen -> menuOpen in NavBarNext (since it can be used for mobile menu or megamenu)

* just use index to key the items

* Use exact versions of @react-aria packages

* Navigation: Make the dropdown header a NavBarMenuItem

* Navigation: Stop using dropdown-menu for styles

* Navigation: Hide divider in NavBarMenu + tweak color on section header

package.json

@@ -243,6 +243,8 @@
     "@opentelemetry/exporter-collector": "0.23.0",
     "@opentelemetry/semantic-conventions": "1.0.0",
     "@popperjs/core": "2.5.4",
+    "@react-aria/focus": "3.5.0",
+    "@react-aria/overlays": "3.7.2",
     "@reduxjs/toolkit": "1.6.1",
     "@sentry/browser": "5.25.0",
     "@sentry/types": "5.24.2",