feat: Enhance MegaMenu with localization, caching, and new properties

- Added localization support for loading states, empty navigation, and submenu actions in both German and English.
- Introduced new properties in IMenuItem for description, openInNewWindow, and external links.
- Updated SPTermStorePickerService to handle cache versioning and language-specific caching.
- Implemented a new MegaMenuCore service for handling term properties, cache management, and navigation URL resolution.
- Refactored TaxonomyNavigationService to build menu hierarchy and filter hidden terms.
- Added automated tests for MegaMenu core functionalities and static asset validation.
- Removed unused ItemDictionary service.
- Created a comprehensive ToDo document outlining the implementation status and future tasks.

src/extensions/megaMenu/loc/en-us.js

@@ -2,6 +2,17 @@ define([], function() {
   return {
     "Title": "MegaMenuApplicationCustomizer",
     "ConfigurationMissing": "Navigation is not configured.",
-    "LoadError": "Navigation could not be loaded."
+    "LoadError": "Navigation could not be loaded.",
+    "Loading": "Loading navigation…",
+    "EmptyNavigation": "The navigation does not contain any visible entries.",
+    "NavigationLabel": "Main navigation",
+    "OpenNavigation": "Open main navigation",
+    "CloseNavigation": "Close main navigation",
+    "OpenSubmenu": "Open submenu for",
+    "CloseSubmenu": "Close submenu for",
+    "SubmenuLabel": "submenu",
+    "ExternalLink": "external link",
+    "Opened": "opened",
+    "Closed": "closed"
   }
 });