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.

src/extensions/currentNavigation/loc/myStrings.d.ts

@@ -0,0 +1,11 @@
+declare interface ICurrentNavigationApplicationCustomizerStrings {
+  Title: string;
+  NavigationLabel: string;
+  ConfigurationMissing: string;
+  LoadError: string;
+}
+
+declare module 'CurrentNavigationApplicationCustomizerStrings' {
+  const myStrings: ICurrentNavigationApplicationCustomizerStrings;
+  export = myStrings;
+}