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/myStrings.d.ts

@@ -2,6 +2,17 @@ declare interface IMegaMenuApplicationCustomizerStrings {
   Title: string;
   ConfigurationMissing: string;
   LoadError: string;
+  Loading: string;
+  EmptyNavigation: string;
+  NavigationLabel: string;
+  OpenNavigation: string;
+  CloseNavigation: string;
+  OpenSubmenu: string;
+  CloseSubmenu: string;
+  SubmenuLabel: string;
+  ExternalLink: string;
+  Opened: string;
+  Closed: string;
 }
 
 declare module 'MegaMenuApplicationCustomizerStrings' {