Add Current Navigation Application Customizer

- Created a new SPFx Application Customizer for current navigation.
- Implemented core functionality to fetch and render navigation items from a SharePoint term set.
- Added styles for the navigation component using SCSS.
- Included localization support for English and German languages.
- Developed unit tests for core navigation functionalities.
- Configured TypeScript and TSLint settings for the project.
- Added validation scripts for static assets and package configuration.

ToDo.md

@@ -0,0 +1,14 @@
+# CurrentNavigation – Umsetzungsstand
+
+Zielversion: 1.0.0
+Status: Implementierung und lokale Tests abgeschlossen; Ship-Build auf Node 8 ausstehend
+
+- [x] SPFx Application Customizer für moderne Seiten
+- [x] Site-Collection-weite Registrierung
+- [x] Termset-Picker über PortalSettings
+- [x] Cache-Dauer, Cache-Version und Debug-Modus
+- [x] CurrentNavigation.Root und CurrentNavigation.Hidden
+- [x] URL-basierte Zuordnung zum aktuellen Web
+- [x] Begrenzbare Navigationstiefe
+- [x] Sicherer DOM-Renderer ohne innerHTML
+- [x] Session-Cache mit Stale-Fallback